Allow removing multiple units through the ECS

The Remove Units call allows removing multiple units by passing an array to be removed.  The ECS was not taking this into account, and now does.

Note: there currently isn't a way to remove ghost units in the UI.  This functionality is tested with a unit test, however, and that work passes.
So, my understanding is that toRemove is a unit name, and command.args[0] is the service name. I think if we're just checking for indexOf we can have a bad match.

e.g. if I have apache2-passenger and apache2 in my environment, and have done stuff with units for both of them, won't the indexOf 'apache2' falsely match for apache2-passenger/0? This isn't a super common situation, to be sure, but we don't know what the local charm name scheme people might use might be.

If we're trying to match service to unit, it's probably safer to split the unit on '/' and match the prefix to the service, so "apache2-passenger/0" becomes "apache2-passenger" which will match the service "apache2-passenger" but not the service "apache2".