Fix context.elevated

The current version of elevated method sets for the original context
the admin role too. This change fix this.

Added unittest.

Closes-bug: #1386932

Change-Id: Ife881112efa151e53bfa4b7af35643dcf2d1114f
(cherry picked from commit 98fae47ad1b9b72e41d444ce6f96cf5f2a3b6f0c)
Merge "Fix context.elevated" into stable/juno