cfg: make reset() clear defaults and overrides
cfg: make reset() clear defaults and overrides

Fixes bug #998396

Both Nova and Keystone need to clear the overrides on their config
object between test runs. It's reasonable to expect the reset()
method would do this, so let's make it so.

Also add a clear() method with the old behaviour.

Change-Id: I192c5bb07e81f0fb844fa2fd429dc2e7133800de

openstack-dev/pbr,JioCloud/pbr,citrix-openstack-build/pbr,klmitch/pbr,citrix-openstack-build/pbr,klmitch/pbr,emonty/oslo-packaging,WiFast/pbr,openstack-dev/pbr,WiFast/pbr,emonty/oslo-packaging,citrix-openstack-build/pbr,redhat-openstack/pbr,JioCloud/pbr,openstack-dev/pbr,varunarya10/pbr,redhat-openstack/pbr,varunarya10/pbr