Added multiple VPC support to ec2_vpc module

Added support for multiple VPCs in the same region provided unique tags are used to differentiate between VPCs.  Using unique tags also allows re-use of the same CIDR blocks, therefore VPC:

```YAML
- name: "Create Virtual Private Cloud (VPC)"

  local_action:
    module: ec2_vpc
    vpc_id_tags: { "Environment":"DEV" }
    state: present
    region: us-east-1
    cidr_block: 10.0.0.0/16
    ...
```
is not the same as:

```YAML
- name: "Create Virtual Private Cloud (VPC)"

  local_action:
    module: ec2_vpc
    vpc_id_tags: { "Environment":"DEMO" }
    state: present
    region: us-east-1
    cidr_block: 10.0.0.0/16
    ...
``` 

*FYI: This is a second attempt to submit a pull request - I messed up my fork and lost changes originally submitted a few days ago.*