Security group change in a VPC should *not* destroy & recreate instance

[wilb]

I've just accidentally destroyed a few instances by adding an extra security group to my config for a few ec2 instances. Luckily they were test boxes! ;-)

This is related to bug #299, however hosts within a VPC can have their security group changed dynamically. I can do this in the AWS console, but when doing so through Terraform it's just hit the "(forces new resource)" condition and as a result dropped & recreated the hosts.

[pmoust]

Dup of #194

[mitchellh]

Closing, dup.