contint: fetch slave scripts on slaves

To assist us in testing, we maintain a bunch of tools, scripts and
wrappers under integration/jenkins.git.  The material there is a
requirement for most of our jobs, unfortunately it is only checked out
on the historical server gallium.

The contint::slave-scripts let us clone the repository under
/srv/slave-scripts on any slave that might need it (aka continuous
integration slaves in production, browsertests in labs).

Change-Id: I0b798d4c896957f2b9eed495c4198eda369d771a
Merge "contint: fetch slave scripts on slaves" into production