Add custom entry point to Camus that can be called by Azkaban2

Add custom entry point to Camus that can be called by Azkaban2

Azkaban2 using the HadoopJava plugin cannot launch a Camus job,
see bug report https://github.com/azkaban/azkaban-plugins/issues/132

This PR adds a method runFromAzkaban() to the CamusJob class that
needs to be specified in the method.run argument in Azkaban job
properties file. The runFromAzkaban() will load the Camus specific
properties from a file and then invoke the regular Camus run()
method.

This is not something we will upstream for obvious reasons but it
does fix the problem.

@yagnik @wvanbergen @airhorns