Cache cascaded nova client

We make the python novaclient as a memeber of ComputeManager, each
time used, we create it only if it's None. This avoids creating the
token for every request.
Note that the python-novaclient can re-auth when the token is expired,
so we need not to do it in application level.

Change-Id: I4a5644264383ffdf863759245086f2a6889152bf