Fix dependencies when setting mesos::master/slave::conf_file

[felixb]

• Allow writing mesos::conf_file to some different place than /etc/default/mesos
• The dependencies to mesos::master/slave::conf_file is already handled in master.pp/slave.pp

[deric]

LGTM, tell me when you're done with all the changes, so that I can do a release :)

[felixb]

I think I'm fine for now.