Use HTTPClient from common apiclient code

Oslo common code contains HTTPClient class which handles sending HTTP
requests to OpenStack servers. It also can handle authentication,
logging and so on. So we can use features from common apiclient instead
of tuskarclient implementation.

Inherited tuskar's HTTPClient from common HTTPClient, removed
duplicated methods, modified _http_request() method to use request()
method from common HTTPClient.
Removed VerifiedHTTPSConnection class - we should use a similar
functionality from common HTTPClient.

Changed an exception processing - `requests` module (which is used in
common code) keeps status in response `status_code` attribute instead of
`status`. So we should use `response.status_code` in from_response()
function from tuskarclient.exc module to get response status.

Added missed requirement - stevedore>=0.12. It's used in apiclient.auth,
module (which is used in tuskarclient.common.http).

Removed unused code from test_http module.

Change-Id: Ib731187661a409a036931aa213278ca9b03a3519
Merge "Use HTTPClient from common apiclient code"