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: Id6bf8b3d5792d26d4cedd8804afec0779a71b19e

.coveragerc

@@ -1,7 +1,7 @@
 [run]
 branch = True
 source = heat,contrib
-omit = */tests/*,heat/openstack/*
+omit = */tests/*
 
 [report]
 ignore_errors = True