Fixes bug 1081975
Nova endpoint in keystone was registered if n-cpu is enabled. However it is
a usual case where n-cpu runs on a different host in multi-node setup and
it results in no endpoint for nova. n-api is a better condition since nova-api
and keystone usually run on a same host.
Change-Id: Ic097e1c3bd30798d9d3c5fb76023fbdb3ae189d9