Change service name to lower case in devstack lib
Change service name to lower case in devstack lib

Service name in Openstack is in lower case. So we should also
do it the same way.

Change-Id: I78ec92c031e8731d9c37fa0bdd70ccaba82427d7

kevin-zhaoshuai/zun,kevin-zhaoshuai/zun,kevin-zhaoshuai/zun