Fixes VPC support when there are name collisions between the security groups

This checks if subnet is set and if so it only looks for a match on the name when the vpc_id is not None.  Otherwise, if subnet is None then it makes sure the vpc_id is also None
Cleaning up formatting.

It looks like whoever added the _get_security_group_ids(...) method  used tabs while the rest of the project uses spaces.  I have converted the function to spaces.

Also cleaned up the placement of subnet_id since it was indented with tabs as well.
Merge pull request #61 from Altonymous/patch-1

Fixes VPC support when there are name collisions between the security groups