Add vpc_group_id and region parameters.

The AWS EC2 API does not accept the 'group_name' parameter when launching
instances into a VPC subnet and requires that the 'group_id' parameter
is used instead.

To prevent confusion, a new parameter 'vpc_group_id' was added to be
used along with parameter 'vpc_subnet_id'.

Mutex and required_together checks are put in place to ensure proper
usage.

Added region as an alternative to ec2_url. (us-west-2 vs. ec2.us-west-2.amazonaws.com)
Mutex added to ensure that only one of the two is used at any given
time.

Updated module docs.
Include private IP and DNS info in module's response json