gerrit: Add file paths filtering support.

Gerrit Trigger allows to restrict triggering of build depending
on which files are modified in the change.

Expose this capability with a new property file-paths.

Also cleanup documentation (omissions and formatting errors).

Change-Id: I4bfc5ab03a10622fdb5730a294037add8c1ad54f
Reviewed-on: https://review.openstack.org/25569
Reviewed-by: Antoine Musso <hashar@free.fr>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins