Support custom params for tron runs from tronctl

[yarrrtem]

Something like this:

tronctl start my_favourite_job date='2013-01-01'

.. but for any custom variable like..

tronctl start my_favourite_mr_job jobconf='cpu-intensive.conf'

Then in the actual job definition, you would specify the variable usage, and a default for the scheduled runs.

run_vars:

• jobconf: 'standard.conf'
  actions:
• my_favourite_job --jobconf=%(RUN_VARS:jobconf) --date=%(shortdate)