elasticsearch: add percent-based shard check

One of the problems that have been observed is the cluster going temporarily
red while elasticsearch shuffles shards around and the nagios check firing a
false positive.

This plugin add functionality to check non-active shards (i.e. everything that
ES reports as not in its place), and critically the ability to check
percentages of the total shards.

This should reduce the number of false positives, the plan is to extend the
plugin with more in-depth checks on the index/shard/etc status and so on.

The plugin by default also checks the cluster status as a whole if it is
non-green, however that can be disabled to avoid false positives for the
reasons explained above.

Change-Id: Icf4240e08dd612ab43825db63b0eae3608b73225
Merge "elasticsearch: add percent-based shard check" into production