Add ability to filter machines list by placement

Filtering in the machine list now supports filtering by the name of charms that are placed on a machine.
(Lets you e.g. find the machine where you placed 'swift' so you can remove it)

This is most helpful on a large MAAS.

if you don't have a real maas handy, test using FAKE_API_DATA thusly:
FAKE_API_DATA=test/fakemaas/landscape/all_ok
% PYTHONPATH=.:$PYTHONPATH python3 bin/openstack-status

(You'll need to have ~/.cloud-install created and readable, and the files ~/.cloud-install/installed and ~/.cloud-install/openstack.passwd must exist and have something (anything ) in them.