Add a privileged user for OpenStack services

Add a privileged user for OpenStack services

Currently Cinder makes all requests to other services (Nova, Swift,
etc.) with current user context. Sometimes Cinder needs privileged
rights for external queries (e.g. asking Nova where an instance is
hosted); there is no way to do it yet.

This patch adds to ability to configure an account with special rights
in the configuration ('os_privileged_user_name',
'os_privileged_user_password' and 'os_privileged_user_tenant' options).
Then, requests that need special permissions can be achieved by creating
a client(privileged_user=True).

Note: This user does not necessarily need to have an admin role
associated with it. For instance, policies can be changed to allow a
specific user (without any roles) to perform special actions.

DocImpact: New configuration options to set a privileged user account
Change-Id: I61d8a6de1c5db5ee2ecce124997f9b6447b04e47