We assume that all groups are present in the inventory, and quite obtuse
errors can result if any are not.

This change adds a precheck that checks for the presence of all expected
groups in the inventory for each service. It also introduces a common
service-precheck role that we can use for other common prechecks.

Change-Id: Ia0af1e7df4fff7f07cd6530e5b017db8fba530b3
Partially-Implements: blueprint improve-prechecks