doc: Document py27 test target
doc: Document py27 test target

Previously we mentioned py27djXX but py27 was not described.

Also add how to run a specific test class or a specific test
in a test class.

Change-Id: If442c29c0fcd9bef207cbda7212a8f1c4b8340d4

ChameleonCloud/horizon,yeming233/horizon,openstack/horizon,NeCTAR-RC/horizon,yeming233/horizon,yeming233/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,BiznetGIO/horizon,noironetworks/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,openstack/horizon,ChameleonCloud/horizon,noironetworks/horizon,noironetworks/horizon,BiznetGIO/horizon,ChameleonCloud/horizon,noironetworks/horizon,BiznetGIO/horizon,openstack/horizon,NeCTAR-RC/horizon,yeming233/horizon,openstack/horizon