Added testr-args option support for tox coverage
Added testr-args option support for tox coverage

So we can check coverage for some test cases like:

  tox -e cover -- heat.tests.test_sqlalchemy_api

Change-Id: Icd721fbced34b4f6b9e550d927d8fcc4c6439ff8

maestro-hybrid-cloud/heat,jasondunsmore/heat,cwolferh/heat-scratch,ntt-sic/heat,gonzolino/heat,srznew/heat,pshchelo/heat,openstack/heat,rh-s/heat,takeshineshiro/heat,dims/heat,maestro-hybrid-cloud/heat,rdo-management/heat,noironetworks/heat,NeCTAR-RC/heat,pshchelo/heat,jasondunsmore/heat,takeshineshiro/heat,openstack/heat,cryptickp/heat,redhat-openstack/heat,steveb/heat,srznew/heat,dragorosson/heat,rdo-management/heat,noironetworks/heat,NeCTAR-RC/heat,gonzolino/heat,dims/heat,miguelgrinberg/heat,cwolferh/heat-scratch,rh-s/heat,steveb/heat,cryptickp/heat,pratikmallya/heat,ntt-sic/heat,miguelgrinberg/heat,pratikmallya/heat,dragorosson/heat,redhat-openstack/heat