Update .coveragerc after the removal of openstack directory
Update .coveragerc after the removal of openstack directory

The openstack directory was used to keep codes from oslo-incubator,
we have retired oslo-incubator,so don't use this directory any more.

Change-Id: Ib80662c6477fd55ecad48600c0d6e1f338383fc8

StackStorm/mistral,StackStorm/mistral,openstack/mistral,openstack/mistral