Juggle shard counts for Cirrus

Two things:
* Lower common's file index to 5 shards to reflect that it is more like
a content index then a 'general' index - it is searched frequently and
we want the searches to be lower overhead.
* Lower the maximum number of shards for a general index from 20 to 10.
This should lower the overhead of searching them at the cost of more overhead
replicating them.  We can live with slower replication.

DEPLOYMENT: This will require an in place reindex of the effected wikis
to take effect.  We'll only actually do commons and let the others come
in whenever we next reindex them which should be safe enough.

Change-Id: Ia72699ff45bc7262ff19f1e503ac102eb8692cd9
Configure cache warmers

Enable cache warming for wikis with more than one shard.  Cache warming
is good for smoothing out I/O spikes caused by merges at the cost of
potentially polluting the cache by adding things that won't be used.

Wikis with more then one shard is a decent way of saying "wikis we expect
will get some search traffic every few seconds".  In this commonet the
term "cache" refers to all kinds of caches: the linux disk cache,
Elasticsearch's filter cache, whatever.

Change-Id: Id3a7013e632c10c94798fb77d36c7dc1542f623a
Merge "Juggle shard counts for Cirrus"