Allow override of key defaults

autorestart defaults to 'unexpected', based on 'exitcodes'.

exitcodes defaults to 0,2.

I need both of these to be different, but may as well make all other common
parameters overrideable at the same time.
Ensure that we clean the supervisor.d dir

... so that we can effectively unmanage processes that were under
supervisor control.

This utilises the salt 'clean: True' technique, where we make the clean
directory require (via require_in) the files that we want to remain in the
cleaned dir. This is logically weird, since naturally these files depend on this
directory. Adding 'makedirs: True' to each of the required files works around this
paradox.
Merge pull request #2 from ministryofjustice/more_config_opts

Allow override of per-service defaults; clean supervisor.d dir