Eanble Datadog monitoring for spark jobserver

Enabled datadog monitoring for spark jobserver
1. Replaced JMX reporting with datadog reporting
2. Added datadog reporting configuration parser
3. Added tests for the configuration parser.
Here is my view on the logic: I feel the logic (of getting the config values) here is kind of complicated. I would prefer: read value for a file. If not-existent, then go to the default value. Try to read value from two files is complicated.