Spark Core - [SPARK-3620] - Refactor of SparkSubmit Argument parsing code

Argument processing seems to have gotten a lot of attention lately, so I thought I might throw my contribution into the ring.  Attached for consideration and to prompt discussion is a revamp of argument handling in SparkSubmit aimed at making things a lot more consistent. The only things that have been modified are the way that configuration properties are read/ processed and prioritised 

Things to note include:
* All configuration parameters can now be consistently set via config file

* Configuration parameters defaults have been removed from the code, and placed into a property file which is read from the class path on startup.  There should be no need to trace through 5 files to see what a config parameter defaults to if it is not specified, or have different default values applied in multiple places throughout the code.

* Configuration parameter validation is now done once all configuration parameters have been read in and resolved from various locations, not just when reading the command line.

* All property files (including spark_default_conf) are parsed by Java property handling code. All custom parsing code has been removed. Escaping of characters should now be consistent everywhere.

* All configuration parameters are overridden in the same consistent way - configuration parameters for sparkSubmit are pulled form the following sources in order of priority
 1. Entries specified on the command line (except from --conf entries)
 2. Entries specified on the command line with --conf
 3. Environment variables (including legacy variable mappings)
 4. $(SPARK_DEFAULT_CONF)/spark-defaults.conf or $(SPARK_HOME)/conf/spark-defaults.conf if either exist
 5. Hard coded defaults in class path at spark-submit-defaults.prop

The existing argument handling is pretty finicky - chances are high that I’ve missed some behaviour - if this PR is going to be accepted/approved let me know any bugs and I’ll fix them up and document the behaviour for future reference
Ok, I've reformatted those comments that were not scaladoc comments