logstash: Update fatal grok pattern

Make the Backtrace clause of the fatal grok pattern be required rather
than optional. Some fatals have been seen in the production logs that
incorrectly parsed the backtrace as being cookie data. In the WMF
configuration all fatals will have backtraces.

Change-Id: If7e8fcf14321f77dd8cb6aa879323b03e09da8e8