Remove nova/openstack/* from .coveragerc

The nova/openstack tree no longer exists since we
don't use oslo-incubator so no need to filter it
out in the coveragerc file.

Change-Id: I504144b2e26aec9a46aa1b415e44c7ab787fe021

.coveragerc

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