Security groups extension for PLUMgrid plugin

This patch includes immplementation of security groups
extension in PLUMgrid plugin.

APIs include CRUD operations for security groups
and security group rules.

Added unit tests.

Implements: blueprint plumgrid-plugin-security-groups
Change-Id: I23fea3fe6c3abbe310e883e00706b93863db3bfb
Merge "Security groups extension for PLUMgrid plugin"