Specify Python 2.7 for cover environment in Tox.
Specify Python 2.7 for cover environment in Tox.

Fixed bug #1123261

Change-Id: I0066176be489a94a99d48bba4b3548ae348809cf

citrix-openstack/build-trove,citrix-openstack-build/trove,changsimon/trove,hplustree/trove,cp16net/trove,zhangg/trove,openstack/trove,changsimon/trove,mmasaki/trove,redhat-openstack/trove,redhat-openstack/trove,cp16net/trove,cp16net/trove,citrix-openstack/build-trove,fabian4/trove,denismakogon/trove-guestagent,hplustree/trove,mmasaki/trove,zhujzhuo/openstack-trove,redhat-openstack/trove,fabian4/trove,citrix-openstack-build/trove,zhujzhuo/openstack-trove,citrix-openstack/build-trove,zhujzhuo/openstack-trove,zhangg/trove,fabian4/trove,mmasaki/trove,citrix-openstack-build/trove,openstack/trove,changsimon/trove,denismakogon/trove-guestagent