monascas is one of several data sources that can be used with watcher.
it is currently deprecated for removal in a future release because
of lack of docs and testing.

the monasca project has recently been confirmed by the tc to be required
due to lack of maintenance.

This task tacks making monasca an optional dep of watcher via extras and
converting the explicit import into conditional imports.

when this is complete it shoudl be possible to install watcher without monasca client
when the monasca backend is not enabled.