Added ability to specify security group for node group

Extended API to support list of security groups in node groups and
node group templates. Changed db schema and provisioning engines (both direct
and heat) accordingly.

Partially implements: blueprint cluster-secgroups
Fixed a ValueError on provisioning cluster with Vanilla plugin.

If the machine is slow, the remote command may output an empty
string. Converting an empty string to an integer results a
ValueError, which caused a failure. This patch attempted to fix
the error by adding a check for the outputted string.
Merge pull request #2 from hongbin/stable/icehouse

Added ability to specify security group for node group