Use stevedore for plugins loading

Stevedore is the common approach to load plugins in OpenStack. It
provides extremely much more options than current handmade plugins
loader. We'll need it espessially for implementing pluggable resources
provisioning code.

In addition, setting vanilla and hdp as default plugins to make minimal
config easier.

Implements blueprint use-stevedore

Change-Id: I96700bf34417c9bcd24d4f60b2deaa88a8c7c2d9