Replaces the instance label on prometheus metrics with the inventory
hostname as opposed to the ip address. The ip address is still used as
the target address which means that there is no issue of the hostname
being unresolvable. Can be optionally enabled or set to FQDNs by
changing the prometheus_instance_label variable as mentioned in the
release notes.

Co-Authored-By: Will Szumski <will@stackhpc.com>
Change-Id: I387c9d8f5c01baf6054381834ecf4e554d0fff35