Fix security group not in the correct VPC

[khuedoan]

Fix #5

Currently the molecule security group is created in the default VPC.

If we use a non-default VPC, for example:

# ...
platforms:
  - name: instance
    image_owner: 099xxx
    image_name: ubuntu/images/xxx
    instance_type: t2.micro
    vpc_subnet_id: subnet-035xxx
# ...

The instance creation will fail because the security group and the instance are in different VPCs:

"msg": "The following group names are not valid: molecule"

Same as ansible/molecule#1644 (merged before we move EC2 driver to its own repository)