Gerrit-2.8: Add secondary index support

This rebuilds secondary indexes on gerrit initial-init and init. They
are optional in gerrit 2.8, and mandatory in gerrit 2.9.

A few common strings are refactored into variables to make this more
concise, and less likely to get fat fingered. If desired, further
symbolic refactoring can be done here as well.

Question remains if we need to make this 2.8 conditional, and how we
can get access to the gerrit revision in this part of puppet.

Partial-Bug: #1082781
Change-Id: Iee94934baaa220313a7e888ba0e2a6530eab0d52
Merge "Gerrit-2.8: Add secondary index support"