Created domain classes for security groups.

Created domain classes for security groups.

Created SecurityGroup and SecurityGroupRule classes so the neutron_utils
for security groups will be returning objects of these types instead of
the OpenStack objects returned by the API calls.

JIRA: SNAPS-116

Change-Id: I76ed1f85f7d54b984fc6f6ac28cee7680a1109e5
Signed-off-by: spisarski <s.pisarski@cablelabs.com>