zuul: switch to run as 'zuul' user BREAKING CHANGE

Zuul has originally been setup on gallium.wikimedia.org to run as the
jenkins user which is created by installing the Jenkins package.  Since
I want to run a Zuul merger on a different host that does not have
Jenkins, lets switch to a dedicated username.

Applying this change will cause havoc. We will have to shutdown Zuul
processes and do several manual tasks:

a) copy the ssh key that lets Zuul communicate with Gerrit:

cp /var/lib/jenkins/.ssh/id_rsa /var/lib/zuul/.ssh/id_rsa

b) fix file ownerships:

sudo chown -R zuul:zuul \
    /var/lib/zuul /var/log/zuul \
    /var/run/zuul /var/run/zuul-merger \
    /srv/ssd/zuul

The git repositories in /srv/ssd/zuul/git will take a while.

c) manually accept Gerrit hostkey:

sudo -u zuul ssh ytterbium.wikimedia.org

d) start zuul and zuul-merger on gallium :)

Change-Id: Ic67983753f7f825ab0246c8de7ee155ae35ceae8