Add support for user/admin permissions in endpoints

Instead of storing the endpoints as a single admin credentials dictionary
(containing the username, password etc.) we should be able to store several
endpoints which can be possibly returned from the deploy object, as well as
the information whether these endpoints have the admin permission or not.
If not, then the deployment is going to be used with the predefined set of
users (that will be implemented next). If there is an admin endpoint, then
the users for benchmarking will be generated (the "admin" role).
This information now gets stored in a dictionary of form:

    "endpoints": [
        {"username": ..., "password": ...,
         "tenant_name": ..., "auth_url": ..., "permission": "admin|user"},
        {"username": ..., "password": ...,
         "tenant_name": ..., "auth_url": ...,
         "permission": "admin|user"},
        ...
    ]

Blueprint benchmarking-with-predefined-users

Change-Id: If36e33e89daa036bd4b0d7db8e2d3b5b5b0f4aca