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
Remove compute api from scheduler driver

Remove unused instance of compute api and belonging import statement
from scheduler driver.

Change-Id: Ice0c3e7555cf4fb00a387c6cc9789515569bea9c
Merge "Remove compute api from scheduler driver"
Merge "Add support for Option Groups in LazyPluggable"