Search specification of for discovery interface.

The format of the query string will be left entirely up to individual `DiscoveryIO` implementations.

We'll be changing the way `ElasticIO` works to just parse the entire query rather than associated the term to the RAW_METRIC_NAME field. But this is a good starting point.
Does this need to be `METRIC_NAME` from https://github.com/rackerlabs/blueflood/blob/master/blueflood-elasticsearch/src/main/resources/metrics_mapping.json ?