fix incorrect specs for attribute merging

in chef, attributes defined at the same precedence levels get deep-merged. attributes
defined at a higher precedence level clobber the attributes at a lower precedence level.
here's an example from a `chef-shell`:

```
chef > node.default.igor = {"a" => "mike", "b" => "joe", "c" => [1, 2, 3]}
 => {"a"=>"mike", "b"=>"joe", "c"=>[1, 2, 3]}
chef > node.override.igor = {"b" => "bob", "c" => [99, 98, 97]}
 => {"b"=>"bob", "c"=>[99, 98, 97]}
chef > node.igor
 => {"a"=>"mike", "b"=>"bob", "c"=>[99, 98, 97]}
```

you can see that the value for `node.igor.c` is NOT `[1,2,3,99,98,97]` but instead just
`[99, 98, 97]`, the value defined at the override level.

the specs here incorrectly stipulate that security groups defined at an override level
should be merged with those specified at the default level, which is incorrect.
fix failing spec for attribute merging
Merge pull request #49 from airbnb/proper_attribute_merging

Proper attribute merging
bump version to v0.7.0
Add support to launch instance into a VPC subnet

Add a rudimentary support for launching instances into VPC. Mainly
this means new vpc_subnet_id argument which specifies the
"subnet-xxx" id of the Amazon EC2 VPC subnet. As the subnet can
live only in one Availability Zone then the availability zone argument
is not needed when launching into vpc. Because of this there's
a new way to specify an array of arguments where only one argument
is required. In addition there's "private_ip_address" option
which can be used to set instance private ip on launch.
Merge branch 'feature/vpc-support2' of github.com:Applifier/stemcell into feature/vpc-support2
Add ec2 ohai hint and fix security_groups to support ids