Add common test base class to hold common things.
Add common test base class to hold common things.

There are several common fixtures that every test case wants. Following
the pattern in Nova, add a common base test case class to hold these things.

Change-Id: I2d2cd91e5051d9cbf230e6f48985d6eddcb7b58a

openstack/networking-bigswitch,stackforge/networking-bigswitch,stackforge/networking-bigswitch,wolverineav/networking-bigswitch,wolverineav/networking-bigswitch,openstack/networking-bigswitch