lanthanum as a jenkins slave

This patch makes lanthanum.eqiad.wmnet a Jenkins slave as requested in
RT #5074.

Sudo:
- hashar is a regular user (no root)
- cleaned up sudo commands, most of them are not needed there.
- User is jenkins-slave on Jenkins slaves

Classes:
- admins::root and admins::jenkins seems to be required
- contint::packages that provides all dependencies needed to run tests
- the role class for Jenkins slave :)

Lanthanum received a SSD drive (RT #5178). It is available under device
name /dev/sdb1 and mounted on /srv/ssd just like on gallium.

Change-Id: I6c9634bad3c326a68f0f9497d814d26e56f54f69
Merge "lanthanum as a jenkins slave" into production