Add document for entry point in setup.cfg
Add document for entry point in setup.cfg

In addition, to remove the contrib/ way to load commands since
it is not implemented at all.

Change-Id: Id15eb10e7cb086d55d2bcad4f719f9120c6f7d1a
Closes-Bug:#1486824

Juniper/python-neutronclient,eayunstack/python-neutronclient,Juniper/python-neutronclient,cryptickp/python-neutronclient,openstack/python-neutronclient,rackerlabs/rackspace-python-neutronclient,openstack/python-neutronclient,eayunstack/python-neutronclient,cryptickp/python-neutronclient,huntxu/python-neutronclient,huntxu/python-neutronclient,rackerlabs/rackspace-python-neutronclient,noironetworks/python-neutronclient,noironetworks/python-neutronclient