Make backends into drivers, loadable as entry points

Added base.py, which has classes for backends to derive from. This is
the stevedore suggested way of writing drivers.
stevedore.readthedocs.org/en/latest/tutorial/creating_plugins.html

Added entrypoints to setup.cfg

implements blueprint backend-abstraction
Change-Id: Ib53bd1ec930f15480e33b9105033e9999f96b842