Configure maximum number of shards per node

Allows cirrus to set index.routing.allocation.total_shards_per_node which
prevents too many shards of the same index from allocating to the same node.
This is really only important for content indexes (because those are the
most highly searched) and probably only really important for really big
wikis.

DEPOYMENT: This does nothing unless configured and then the index update
script must be run.

Change-Id: I83a9a4b23c3edb2cd177dfccdd7e650598ee2a22
(cherry picked from commit 839e5f876892ac652ee47b24a5a7d1426c2de26a)
Merge "Configure maximum number of shards per node" into wmf/1.24wmf20