Support specifying replica count per index type

We'll want to have four way replication for preformance reasons for enwiki
in production but only for the content index.  The general index can keep
the 3 way replication.

Change-Id: I65193ae6ef730a898a6a7fdb14f764056fb03646
(cherry picked from commit 76c3c0e60c446b534cd04cdf0c75e3c8c79dfcc5)