Cleanup nova cell configuration before config it
Cleanup nova cell configuration before config it

Some old configuration(such as, LOG_COLOR config) will remain
if we don't cleanup.

So, we should cleanup the configuration before we config it.

Change-Id: I7aff609dadf3acba13a36894614b35005f51280d

dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,openstack-dev/devstack,group-policy/devstack,dtroyer/devstack,group-policy/devstack,nawawi/openstack