Make it easy to override the marathon framework user

Currently the module hardcodes the systemd service user to "marathon".
With this change one would not need to create an extra systemd config to
override the user.

So why would one need to override the Marathon user? Some apps require
root access to run. You can't run those with Marathon unless you
override the default user to root. Marathon also provides a
`--mesos_user` command line flag which allows you to run apps using
arbitrary users. You need to run the framework as root to enable this
functionality.