ec2_group: make group_name only match groups in specified VPC

ec2_group allows specification of another group by name with group_name. However, it currently matches names against groups in all VPCs, even if the currently-being-managed security group has a vpc_id specified.

When referring to other security groups with group_name, it is only possible to reference those in the same VPC as the currently-being-managed group (or, for a non-VPC EC2 security group, another such group). This PR brings ec2_group in line with that restriction.