hn1 was deprecated for a while and this patch removes it altogether. The
support code in Cluster.hs is also removed.

This is a tool that checks how many instances (of same size, specified
by command line arguments) can be added to a cluster while remaining N+1
compliant.

This patch makes sure that all references to the name of the software is
ganeti-htools, not simply htools.

This patch adds a QuickCheck-based test infrastructure and initial tests
based on it. The PeerMap module has a 100% coverage ☺️

Side-note: one has to read the source of QuickCheck to see how to use it
(especially the Batch submodule), the docs are not enough…