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
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 0ec9f88..a03cc1c 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -323,6 +323,13 @@
     greedy matchers and to escapes dots!
     Example: ``email_filter: ^.*?@example\.org$``.
 
+    *username_filter*
+    This is used for any event.  It takes a regex applied on the performer
+    username, i.e. Gerrit account name.  If you want to specify several
+    username filters, you must use a YAML list.  Make sure to use non greedy
+    matchers and to escapes dots!
+    Example: ``username_filter: ^jenkins$``.
+
     *comment_filter*
     This is only used for ``comment-added`` events.  It accepts a list of
     regexes that are searched for in the comment string. If any of these