TabletBalancer has an implicit requirement for a specific constructor

TabletBalancers are an extension point designed for non-core-Accumulo devs to implement balancers for a specific purpose.

As noticed in ACCUMULO-3356, there is an unwritten requirement that any concrete implementations of TabletBalancer have a constructor that takes a String object as an argument. This wouldn't be so bad but it's a silent failure that falls back to the DefaultBalancer (that's why we haven't noticed ACCUMULO-3356 for ages).

TabletBalancer already has an init method which could be used. There are lots of other ways it could be addressed. At a minimum, this should be prominently documented.