Add CPU utilization and file descriptor count to JvmMetrics

With the addition of multithreading in Samza, we've seen more jobs become bottlenecked on CPU. In order to help trouble shoot these cases, we should add the following metrics to JvmMetrics.scala:

process-cpu-usage - com.sun.management.OperatingSystemMXBean#getProcessCpuLoad()
system-cpu-usage - com.sun.management.OperatingSystemMXBean#getSystemCpuLoad()
open-file-descriptor-count - com.sun.management.UnixOperatingSystemMXBean#getOpenFileDescriptorCount()
thread-count - java.lang.management.ThreadMXBean#getThreadCount()

The first three metrics will only be emitted if the bean returned by ManagementFactory.getOperatingSystemMXBean() is of the appropriate type.

All of the new metrics should be gauges, not counters.