Bank plots use long node names

[aphyr]

It'd be nice if we could pick shorter node names in plots like jepsen.tests.bank/plot; on ec2, for instance, you get legend names like "ec2-534-23-23-212.compute-1.amazonaws.com", which could be much shorter. I think there's some code for finding or removing common prefixes and/or postfixes in jepsen (maybe jepsen.util?) that could be handy here.