EC2 instance SG not included in RDS instance's SG

[milosgajdos]

When creating CF stack which has EC2 instance(s) and RDS, RDS SG does not include EC2's SG.
This has a consequence of EC2 instance not being able to communicate with RDS instance.

This is a similar issue to #13

[ashb]

@mattmb Was this also fixed in #51 – i.e. can we now specify a SG of the EC2 instances by reference to allow from the RDS?

[mattmb]

Don't think it is because I don't think we've added a way to specify the SGs in the RDS section of the yaml.