Fix tests with testtools>=0.9.39

'rpc_backend' config option in
ceilometer.openstack.common.rpc.__init__.py defaults to
'%s.impl_kombu' % __package__. Because the RPC module is imported
in some other files as ceilometer.openstack.common.rpc, rpc_backend
is set to 'ceilometer.openstack.common.rpc.impl_kombu'. But when
the test loader tries to import the RPC module as openstack.common.rpc
(since it is starting inside the ceilometer folder), registering the
option fails, because we try to register a different default value.

The issue surfaced because testtools==0.9.39 started checking __init__.py
files as well. It is fixed by passing a top-level directory parameter to
the test discovery process.

Change-Id: I08036b54def3eb496772bcf183927b9f70bb7a86
Closes-bug: #1365613