This adds a --strict option that can be passed in when calling the
nova-manage cell_v2 discover_hosts command. When this option is used,
the command will only return success if a new host has been found.
In any other case it is considered a failure.
Closes-Bug: #1692982
Change-Id: I942af11a3987e1edce67423c66931ad9ece65587
aaae213bf3