Its difficult to discern which unit is a leader in amulet

[lazypower]

Often times when I'm working with quorem or need special behaviour out of a "leader" unit, I use leader-election to do things, like spin up a management service, or deploy specialized configuration.

I will often need to test this in amulet, and thus far the only way i've come up with to find the leader is basically sentry.run('is-leader') in a loop and break when we find the leader.

What would be handy is to have this exposed in amulet with a get_leader('service') so I can scope assertions without implementing the same for loop in all of my amulet tests.

[lazypower]

This has a line item in #116 and is somewhat a duplicate of it