[EXPERIMENTAL] Add function to get ec2 metadata to inventory helper

You can get ec2 metadata like this.

```ruby
pp host_inventory[:ec2]

{"ami-id"=>"ami-29dc9228",
 "ami-launch-index"=>"0",
 "ami-manifest-path"=>"(unknown)",
 "block-device-mapping"=>{"ami"=>"/dev/xvda", "root"=>"/dev/xvda"},
 "hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "instance-action"=>"none",
 "instance-id"=>"i-fdcea4e4",
 "instance-type"=>"t2.micro",
 "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "local-ipv4"=>"172.30.1.248",
 "mac"=>"06:9f:24:59:0d:08",
 "metrics"=>{"vhostmd"=>"<?xml version=\"1.0\" encoding=\"UTF-8\"?>"},
 "network"=>
  {"interfaces"=>
    {"macs"=>
      {"06:9f:24:59:0d:08"=>
        {"device-number"=>"0",
         "interface-id"=>"eni-10830467",
         "ipv4-associations"=>{"54.64.124.181"=>"172.30.1.248"},
         "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
         "local-ipv4s"=>"172.30.1.248",
         "mac"=>"06:9f:24:59:0d:08",
         "owner-id"=>"853708250916",
         "public-hostname"=>"",
         "public-ipv4s"=>"54.64.124.181",
         "security-group-ids"=>"sg-597bbc3c",
         "security-groups"=>"launch-wizard-3",
         "subnet-id"=>"subnet-6654bd11",
         "subnet-ipv4-cidr-block"=>"172.30.1.0/24",
         "vpc-id"=>"vpc-3108e254",
         "vpc-ipv4-cidr-block"=>"172.30.0.0/16"}}}},
 "placement"=>{"availability-zone"=>"ap-northeast-1b"},
 "profile"=>"default-hvm",
 "public-hostname"=>"",
 "public-ipv4"=>"54.64.124.181",
 "public-keys"=>
  {"0"=>
    {"openssh-key"=>
      "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCBgoqZg320DAgdessGJXTadkkQ9HHd552na9nERPiOU36WU1DE83CHDrCi7rToPLji0M+l1Vi8yXUrqPkTeC2AY7HpbG2aY+7ReLtsZin+gB2+oEMk3UGQP29foG3AB6syy7n9lm71qz9q91tItaBMszVNH7oRC1a7ZB8eJx6hNAid8uD8zsvmldOLZ+rCWTRBJLbjEm3AYTFfduL/1wptA+/7u7CJ8rRCxg4Dfwja0APTZeyjL31w3AraGTlQL0IxxTjogVb1AoGbpWC8RjUfV/IAGJGntpIJF1J+tayDSCIpvic9oi9DqGmijZpnwisdVCn88o9yL9vJ7Wpj4iDt miya\n"}},
 "reservation-id"=>"r-66490760",
 "security-groups"=>"launch-wizard-3",
 "services"=>{"domain"=>"amazonaws.com"}}
{"ami-id"=>"ami-29dc9228",
 "ami-launch-index"=>"0",
 "ami-manifest-path"=>"(unknown)",
 "block-device-mapping"=>{"ami"=>"/dev/xvda", "root"=>"/dev/xvda"},
 "hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "instance-action"=>"none",
 "instance-id"=>"i-fdcea4e4",
 "instance-type"=>"t2.micro",
 "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "local-ipv4"=>"172.30.1.248",
 "mac"=>"06:9f:24:59:0d:08",
 "metrics"=>{"vhostmd"=>"<?xml version=\"1.0\" encoding=\"UTF-8\"?>"},
 "network"=>
  {"interfaces"=>
    {"macs"=>
      {"06:9f:24:59:0d:08"=>
        {"device-number"=>"0",
         "interface-id"=>"eni-10830467",
         "ipv4-associations"=>{"54.64.124.181"=>"172.30.1.248"},
         "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
         "local-ipv4s"=>"172.30.1.248",
         "mac"=>"06:9f:24:59:0d:08",
         "owner-id"=>"853708250916",
         "public-hostname"=>"",
         "public-ipv4s"=>"54.64.124.181",
         "security-group-ids"=>"sg-597bbc3c",
         "security-groups"=>"launch-wizard-3",
         "subnet-id"=>"subnet-6654bd11",
         "subnet-ipv4-cidr-block"=>"172.30.1.0/24",
         "vpc-id"=>"vpc-3108e254",
         "vpc-ipv4-cidr-block"=>"172.30.0.0/16"}}}},
 "placement"=>{"availability-zone"=>"ap-northeast-1b"},
 "profile"=>"default-hvm",
 "public-hostname"=>"",
 "public-ipv4"=>"54.64.124.181",
 "public-keys"=>
  {"0"=>
    {"openssh-key"=>
      "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCBgoqZg320DAgdessGJXTadkkQ9HHd552na9nERPiOU36WU1DE83CHDrCi7rToPLji0M+l1Vi8yXUrqPkTeC2AY7HpbG2aY+7ReLtsZin+gB2+oEMk3UGQP29foG3AB6syy7n9lm71qz9q91tItaBMszVNH7oRC1a7ZB8eJx6hNAid8uD8zsvmldOLZ+rCWTRBJLbjEm3AYTFfduL/1wptA+/7u7CJ8rRCxg4Dfwja0APTZeyjL31w3AraGTlQL0IxxTjogVb1AoGbpWC8RjUfV/IAGJGntpIJF1J+tayDSCIpvic9oi9DqGmijZpnwisdVCn88o9yL9vJ7Wpj4iDt miya\n"}},
 "reservation-id"=>"r-66490760",
 "security-groups"=>"launch-wizard-3",
 "services"=>{"domain"=>"amazonaws.com"}}
{"ami-id"=>"ami-29dc9228",
 "ami-launch-index"=>"0",
 "ami-manifest-path"=>"(unknown)",
 "block-device-mapping"=>{"ami"=>"/dev/xvda", "root"=>"/dev/xvda"},
 "hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "instance-action"=>"none",
 "instance-id"=>"i-fdcea4e4",
 "instance-type"=>"t2.micro",
 "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
 "local-ipv4"=>"172.30.1.248",
 "mac"=>"06:9f:24:59:0d:08",
 "metrics"=>{"vhostmd"=>"<?xml version=\"1.0\" encoding=\"UTF-8\"?>"},
 "network"=>
  {"interfaces"=>
    {"macs"=>
      {"06:9f:24:59:0d:08"=>
        {"device-number"=>"0",
         "interface-id"=>"eni-10830467",
         "ipv4-associations"=>{"54.64.124.181"=>"172.30.1.248"},
         "local-hostname"=>"ip-172-30-1-248.ap-northeast-1.compute.internal",
         "local-ipv4s"=>"172.30.1.248",
         "mac"=>"06:9f:24:59:0d:08",
         "owner-id"=>"853708250916",
         "public-hostname"=>"",
         "public-ipv4s"=>"54.64.124.181",
         "security-group-ids"=>"sg-597bbc3c",
         "security-groups"=>"launch-wizard-3",
         "subnet-id"=>"subnet-6654bd11",
         "subnet-ipv4-cidr-block"=>"172.30.1.0/24",
         "vpc-id"=>"vpc-3108e254",
         "vpc-ipv4-cidr-block"=>"172.30.0.0/16"}}}},
 "placement"=>{"availability-zone"=>"ap-northeast-1b"},
 "profile"=>"default-hvm",
 "public-hostname"=>"",
 "public-ipv4"=>"54.64.124.181",
 "public-keys"=>
  {"0"=>
    {"openssh-key"=>
      "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCBgoqZg320DAgdessGJXTadkkQ9HHd552na9nERPiOU36WU1DE83CHDrCi7rToPLji0M+l1Vi8yXUrqPkTeC2AY7HpbG2aY+7ReLtsZin+gB2+oEMk3UGQP29foG3AB6syy7n9lm71qz9q91tItaBMszVNH7oRC1a7ZB8eJx6hNAid8uD8zsvmldOLZ+rCWTRBJLbjEm3AYTFfduL/1wptA+/7u7CJ8rRCxg4Dfwja0APTZeyjL31w3AraGTlQL0IxxTjogVb1AoGbpWC8RjUfV/IAGJGntpIJF1J+tayDSCIpvic9oi9DqGmijZpnwisdVCn88o9yL9vJ7Wpj4iDt miya\n"}},
 "reservation-id"=>"r-66490760",
 "security-groups"=>"launch-wizard-3",
 "services"=>{"domain"=>"amazonaws.com"}}
```