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.
Remove commented-out sections

These may be important if we want to submit a PR back upstream, but for
us they're just clutter.
Restart nerve and synapse if they crash

If the zookeeper cluster disappears, nerve and synapse should be
resilient. Unfortunately, the nerve and synapse executables themselves
will crash in this case, but at least we can get a bit of this by
teaching Systemd to restart the services.
Wait 2 seconds before restarting Nerve / Synapse

To prevent the services from blowing through their restart limit (5
restarts in 10 seconds), this adds a 2 second restart delay.