[RDS] VPC Security Group Support

This pull request merges in #1602 from @pchico and adds:

 * Rename `vpc_security_groups_ids` -> `vpc_security_groups` for consistency with existing API
 * Allow passing in EC2 SecurityGroup objects similar to how DBSecurityGroups work
 * Documentation updates
 * Unit tests for passing in both strings and SecurityGroup objects

@toastdriven please review.