pin to specific plugin versions
reorder variables, comment about $JENKINS_VERSION
disable admin monitors
Scale Jenkins without the use of build slaves

This commit removes configuration for the Mesos Slaves plugin, and
instead uses executors on each of the masters. It reduces the complexity
of the environment by allowing one to horizontally scale the Jenkins
masters using Marathon.

The desired use-case here is that the Jenkins masters will be running
_something_ that results in the build taking place elsewhere. For
example: this might be Beaker running acceptance tests in EC2 or
vmpooler.

This commit also begins managing configuration for the Jenkins Logstash
plugin. There are enough changes here that this could have probably been
two commits, but eh.
use wget -nc

So we don't re-download everything when trying to test this script
locally.