Reduce API client object in a request
Reduce API client object in a request

Mistralclient is created every time methods in 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.

Change-Id: I46f101505801f8d650722c9c84397ab4d813e097

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard