Add support for Option Groups in LazyPluggable
Add support for Option Groups in LazyPluggable

Use @markmc's suggestion to enhance LazyPluggable with an
optional config group. Also fix the baremetal database
backend option to use the "baremetal" config group.

Fixes LP #1093043

Change-Id: I28cf51a2962f516fcef4ced19e30c985220e86dc

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects