Delegate authentication to quantumclient

Quantumclient can deal with authentication, getting the token and
refreshing it when needed. There should be no need for nova to do
those explicitly. Additionally nova can save some time not having
to recreate the Client object on each request.

Fix a couple of places that relied on the exceptions module being
imported inside quantumv2.

Part of blueprint fewer-networking-token-checks
Fixed bug 1177579

Change-Id: I007cef1f0bd688036fa45d79792e6e350c145f05
Merge "Delegate authentication to quantumclient"