Allow pipelines triggers to filter by username

Currently we can filter pipeline triggers by email address but not
username. This is an issue for users that have no email addresses
such as Jenkins.

This patch adds a new filter "username_filter" to the gerrit trigger
section.

Change-Id: I66680ab7e9e5ff49466269175c8fb54aef30e016
Merge "Allow pipelines triggers to filter by username"