Always pass a project_id

Since v1.1, the `X_PROJECT-ID` became a required header. This patch adds
a way to specify the project id regardless of the backend. In order to
pass the project id, it's necessary to pass the `os_project_id` option
in the `auth_opts` dictionary. For example:

  auth_opts = {'options': {'os_project_id': 'yadayada'}}

Partially-Implements blueprint: api-v1.1

Change-Id: If72b758e634dc1e6bc1e1cc0c63361e8a43861bb