Adding support for using vpc subnets.

This change introduces parameter dependencies when using a vpc subnet in
that security group identifiers must be passed as ID's instead of names,
and at least one security group is required in order to create an
instance in a vpc subnet.
Merge remote-tracking branch 'upstream/master'
Lookup security group ids by name
Merge pull request #49 from calebfornari/master

Add VPC support