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
Merge "Configure maximum number of shards per node"