Fixed a bug in the slave that crashed it when
hitting /statistics/snapshot.json.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/9748

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/branches/0.12.x@1454635 13f79535-47bb-0310-9956-ffa450edef68
Fixed a bug in resource monitor to dispatch to IsolationModule::usage()
instead of invoking usage() directly.

Review: https://reviews.apache.org/r/9834

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/branches/0.12.x@1454636 13f79535-47bb-0310-9956-ffa450edef68
Fixed the monitor test to spawn the isolation module.

Review: https://reviews.apache.org/r/9836

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/branches/0.12.x@1454637 13f79535-47bb-0310-9956-ffa450edef68