Allow omnibus-ctl to be invoked without having the service management commands.

Addons (e.g. pushy/reporting/manage) don't manage their own services but use the private-chef-ctl runit.

This patch allows for a second initialization option 'service_commands' (true by default to mimic current behaviour) where the ctl command does not have any runit related commands.  Includes updated tests

Note:  This doesn't include a new wrapper script ala. omnibus-ctl - I'm dropping off a script setting the options directly in the relevant omnibus software config (e.g. omnibus-reporting-ctl)

cc @christophermaier, @schisamo