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.