Sahara-tests don't support http and https proxies

[Rafik HARZI]

When sahara-tests are run in closed environment, where they need a proxy to go out and use openstack public endpoints, they can't reach the endpoints because they try to communicate directly with them without going through the proxy.

Errors are the following:

urllib3.connectionpool [-] Retrying (Retry(total=0, connect=None, read=None, redirect=5)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fb59e1fbc50>: Failed to establish a new connection: [Errno 110] Connection timed out',)': /v2.0/tokens

<class 'urllib3.exceptions.MaxRetryError'>

[Rafik HARZI (rafik.harzi)]

apparently it's related to this tempest bug: https://bugs.launchpad.net/tempest/+bug/1556864.
Since sahara-tests uses tempest to initialize the http object. And the http object created doesn't handle proxying.