Resolves ryu plugin unittest errors

Removed obsolete DATABASE value tests in ryu/test_defaults.py
Added fake_ryu client in test_ryu_security_group.py
Imported quantum.plugins.ryu.db.models_v2 in test_ryu_plugin.py

Fixes bug #1151642

Change-Id: I60e5aee90da9fa32bd285e6f6bbd95cd7ee1ce2c