Add interface for listing security groups of an instance

This is already available in nova but not exposed via client.

* novaclient/v1_1/servers.py:
   New interface to list security groups of an instance.

* novaclient/v1_1/shell.py:
   New sub command list-secgroup.

* novaclient/tests/v1_1/fakes.py,
  novaclient/tests/v1_1/test_servers.py,
  novaclient/tests/v1_1/test_shell.py:
   Add corresponding tests.

Implements: blueprint servers-list-secgroup

Change-Id: I505bcffdbb15b84bfd73cae5ef5a8fb9c69bd7b9
Merge "Add interface for listing security groups of an instance"