erp5-cluster: Assign cluster-unique names to zope processes.

Also, do not tolerate missing instance-count internal zope partition
parameter.
erp5-cluster: Make a few variable names clearer.
components: Add libuserhosts.
erp5: Add support for auto-generated hostnames.

Implemented using libuserhosts. Only provided to zope processes so far, may be
extended in the future with each process seeing different sets of aliases,
with potentially different resolutions.