Fix auth issue in get_instance_host

_get_instance_host was attempting to use the
admin username and password with the user's
tenant, which leads to 401 auth errors from
Keystone when attempting to determine the
instance host from nova.

This patch reads the admin tenant from the same
place as the username and password and uses that
instead. It also removes the keystone call that
was used to determine the tenant_id from the name,
since that is no longer needed.

DocImpact grizzly

Change-Id: If7ab8df24f8a727b458a780af8909e4cc6e1d0eb
Fixes: bug 1202462
Merge "Fix auth issue in get_instance_host" into stable/grizzly