Make default time span of ES index configurable

[yurishkuro]

Right now ES storage rotates indices every 24hrs because of this code in es span writer:

func indexNames(span *model.Span) (string, string) {
	spanDate := span.StartTime.Format("2006-01-02")
	return spanIndexPrefix + spanDate, serviceIndexPrefix + spanDate
}

It would be better to have a CLI flag to configure the time span of the index, and truncate span.StartTime to that granularity.

[pavelnikolov]

@yurishkuro I would also suggest allowing custom format.

At the moment all time based indices that we have in our ES clusters are named like <index-name>_YYYY.MM.DD. And we have a single cronjob which deletes indices with data older than some configured number of days. Jaeger indices are the only exception to this naming convention and we have to run/maintain a separate cronjob just for Jaeger. Not a big deal, but it's a little annoying : )

[pavolloffay]

Please see #840 and rollover api issue