Create an option for Spark path

Remove the hard-coded path to the Spark installation and move it
to an option. The default is '/opt/spark', where Spark is
installed by the DIB.

Co-Authored-By: Trevor McKay <tmckay@redhat.com>

Change-Id: I428f0148e893695a2a225006f0fe5c8bbb9b972f
Merge "Create an option for Spark path"