gallium: let ci folks sudo as jenkins-slave

The Jenkins jobs used to run as "jenkins". They are now run as a
separate user ("jenkins-slave") in order to let CI folks debug things and
run maintenance scripts, need similar rights to be able to interact with
the Jenkins slave files on gallium.

Change-Id: I60c7761c6b49dc946ff3bdce3c02261a1b70c90c
Merge "gallium: let ci folks sudo as jenkins-slave" into production