Update SmartStack cookbook for CentOS 7 + Brigade

1. We install ruby via the rbenv cookbook's 'system' recipe, not the
   ruby cookbook
2. RHEL 7 / CentOS 7 force our hand into using SystemD as an init
   system (instead of runit).
3. Use the 'sudo' cookbook instead of manually creating a
   /etc/sudoers.d/ file.
4. Remove some ancillary files in the cookbook which don't seem to be
   used.