Fix exception being thrown for new instance

Currently if we are trying to register an instance that has not yet
been registered with the elb an exception is thrown.
Adds the ability to restore a db snapshot to a new instance
Allow for changing of security group of RDS in VPC

The code used to rely on the "subnet" argument
being set to determine if this instance was within
a VPC and switch the security group parameter from
secutiry_groups to vpc_security_groups. This works
fine for the command=create case.

However in the command=modify case, the "subnet"
argument is considered invalid (As the AWS API
does not allow for subnet modification of a
provisioned instance). This results in the
security_groups parameter always being treated as
a non-VPC security group, even when it is.

The underlying boto library uses two separate
arguments for when the security group is part of a
vpc (vpc_security_groups) and for when it is not
(security_groups). I feel replicating this
 separation is probably the simplest and most
explicit way of solving this issue.

This pull request implements the separation.