Add support for configurable cinder backends

Adding support to cinder.conf for configuring alternate backends. This
will allow us to back cinder with something like a SAN. Additionally,
as some SAN's may not have native cinder support, add the ability to
install alternate python packages.
It could be, but I preferred that it be explicit.  If we ever decide to change it after the fact, it would need to be changed in perhaps a less than obvious place.  Also, we may not want a default backend.