api: allow client to pass :api_key and :api_secret on a per call basis

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
http: add support for JSESSIONID

The JSESSIONID is now extracted and saved in the Resource(s) object
(along with the response and etag).

The client can then cache it and specify it in the options hash for
subsequent calls, instead of :username and :password.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
models: add support for basic tenant operations

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>