VPC security group membership fix for boto.

* facts now provides a list of existing vpc security group memberships
* adding a comma separated list of sg-* tags properly modifies membership
Merge branch 'vpc_fix' of https://github.com/ebrevdo/ansible into ebrevdo-vpc_fix
Some cleanup for the rds/vpc_security_groups fix
Merge branch 'ebrevdo-vpc_fix' into devel