Fix field fetching for index patterns with an interval

Prior to this change, if you tried to create an index pattern based on event times, it was only possible to do this if you had indices that matched the pattern within +/-5 intervals. (For example, if I was trying to create a daily-based index pattern, it wouldn't work if all of my indices were more than 5 days ago or 5 days into the future).

This change modifies the way we look up fields. Instead of generating the expected index pattern names (such as logstash-2014-12-05, logstash-2014-12-16, etc.) and asking ES if those patterns match existing indices, then looking up fields in the matching indices, we query ES for existing indices that match the pattern, and look up fields from those instead.

Closes #1950.