Lookup Security Groups by name using filter group-name, for non-default VPCs

Asgard is unable to find Security Groups in accounts that are EC2-VPC where you're using a non-default VPC.

Issues #336 and #329 refer to a few problems with security groups and multiple VPCs.  This doesn't fix all the issues, (there are still issues when using the same name across VPCs, for instance) but it is a very easy fix to making Asgard work at all with non-default VPCs.

Basically rather than using GroupName with DescribeSecurityGroups, whose behavior varies between EC2-Classic and EC2-VPC, use a filter "group-name" instead.