Add AllocationDecider that takes free disk space into account

This commit adds two main pieces, the first is a ClusterInfoService
that provides a service running on the master nodes that fetches the
total/free bytes for each data node in the cluster as well as the
sizes of all shards in the cluster. This information is gathered by
default every 30 seconds, and can be changed dynamically by setting
the `cluster.info.update.frequency` setting. This ClusterInfoService
can hopefully be used in the future to weight nodes for allocation
based on their disk usage, if desired.

The second main piece is the DiskThresholdDecider, which can disallow
a shard from being allocated to a node, or from remaining on the node
depending on configuration parameters. There are three main
configuration parameters for the DiskThresholdDecider:

`cluster.routing.allocation.disk.threshold_enabled` controls whether
the decider is enabled. It defaults to false (disabled). Note that the
decider is also disabled for clusters with only a single data node.

`cluster.routing.allocation.disk.watermark.low` controls the low
watermark for disk usage. It defaults to 70.0, meaning ES will not
allocate new shards to nodes once they have more than 70% disk
used. It can also be set to an absolute byte value (like 500mb) to
prevent ES from allocating shards if less than the configured amount
of space is available.

`cluster.routing.allocation.disk.watermark.high` controls the high
watermark. It defaults to 85.0, meaning ES will attempt to relocate
shards to another node if the node disk usage rises above 85%. It can
also be set to an absolute byte value (similar to the low watermark)
to relocate shards once less than the configured amount of space is
available on the node.