Disk and Memory Monitor Plugin

At present to get the actual utilization on the compute only the CPU plugin in available. There are various occasion when the admin would like to know the actual utilization (disk/memory) on the compute. With this feature we would like to introduce 2 new monitor plugins one for disk and other for memory to report the actual usage statistics. These statistics will be dumped into compute_nodes.metrics and can be fetched from the API. Also the scheduler changes to understand these metrics will be included as part of this feature.