Add cgroup memory limits to the jenkins_slave module. Nodes including
jenkins_slave::cgroups will have the jenkins user's processes limited
to use only 90% of the nodes available memory. There are two
exceptions to this. The jenkins process run by the jenkins user will
not have its memory capped. Also there is a soft memory limit of
total physical memory * 0.75. If there is memory contention the
jenkins user processes may get only 75% of available memory.
Change-Id: If4a6439b536df4003d74fd1a93686c1c2401ae20
1467f69191