Reuse API client object in a request

Previously API client object like novaclient are created
every time methods in openstack_dashboard/api.
It leads to regeneration of API HTTP connections.
This commit caches API client object for a WSGI request
to reduce the number of HTTP connection to backends.
As WSGI request object is recreated, a cached API client
object will live in a single request to Horizon.

Change-Id: I66620b3b0a2737c7232d91006e65d0fabbd3090b
Closes-Bug: #1352144
Merge "Reuse API client object in a request"