Allow setting the timezone for sprintf in Logstash event class

A regular problem that arises for users is that Logstash uses UTC time everywhere. Although this is suitable and usually highly desirable for timestamps, there are other places where using UTC doesn't make sense and leads to unexpected results for end-users.

One particular example is with the elasticsearch output plugin as discussed here:

https://github.com/logstash-plugins/logstash-output-elasticsearch/issues/165

Allowing for example, an option to change the default timezone for everything but timestamps would result in a better experience for the end-user, many of whom expect the (in reference to the issue above) indexes to be rotated on local time rather than UTC and better integration with other tools/services on the server, like cron (and curator called through cron), which usually operate in local time.

The following branch seems to have the beginnings of a fix for this:

https://github.com/elastic/logstash/compare/elastic:master...talevy:sprintf-with-tz