contint: /srv/localhost should belong to jenkins-slave

We used to run tests as 'jenkins' user. That has been changed to a
jenkins-slave user to accomodate with production slave not having
jenkins installed and prevent test from accessing 'jenkins'.

/srv/localhost was still using 'jenkins' user which is some leftover
cruft.  Switch it to jenkins-slave which would let the manifest pass on
lanthanum.eqiad.wmnet.

Fix 04d331f992

Change-Id: If5c4ac5a04aac3c00f6988f74b333e9c1df5bc23