EC2 Security Group names are unique given a VPC. When a group_name
value is specified in a rule, if the group_name does not exist in the
provided vpc_id it should create the group as per the documentation.
The groups dictionary uses group_names as keys, so it is possible to
find a group in another VPC with the name that is desired. This causes
an error as the security group being acted on, and the security group
referenced in the rule are in two different VPCs.
To prevent this issue, we check to see if vpc_id is defined and if so
check that VPCs match, else we treat the group as new.
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