Support for benchmarking with existing users (part 1)

This patch covers next topics:

1) Changes db deploymnet model to store admin endpoint and list of
   non-admin users

2) Changes objects.deployment to have this admin and users instead of
   endpoints

3) Changes input format of ExistingCloud engine, to make it easy to
   pass users & admins

4) Changes engine.bind method to accept admin and list of users

5) Code cleanup related to removing list of endpoints and making
   admin/non-admin stuff

6) Fix CLI code related to deployment model change

7) Fix docs & samples

8) Fixing all related tests

In next patch we should drop default "users" context and use users
from deployment if they are passed.

Change-Id: Ifb469d80c61ee5f26f313db75c98a6d496bcdb92
Merge "Support for benchmarking with existing users (part 1)"