Merge lp://staging/~pwlars/charms/trusty/jenkins-remote-slave/jenkins-new-slave-pkg into lp://staging/~canonical-ci-engineering/charms/trusty/jenkins-remote-slave/trunk

This uses an updated version of the jenkins-slave package, and allows for different units to run on the same host, but use a different JENKINS_RUN directory so that the slave.jar can be different for each unit. This wasn't previously important because we had a lot of nodes connected to just a few jenkins master running the same version of jenkins. Today though, we have lots more jenkins masters that have no guarantee of keeping to the same version. We also have a lot greater chance of simultaneous restarts causing a collision when downloading the .jar file. This should prevent all of those problems.