Reduce number of novaclient calls

Currently, each client creates a new session for
each call. This fix makes novaclient re-use
sessions in the most obvoius cases.

Conflicts:
	openstack_dashboard/api/nova.py

Partial-Bug: #1247056

Change-Id: Ie99ecb66304cf40e4f5fdd31fab5162ed11b863e
(cherry picked from commit ddc479272f5402ff778c45892acc3ac7613b7c11)
Merge "Reduce number of novaclient calls" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/horizon into stable/havana_routerrules