Add an initscript for sentinel on apt-based systems

[cdent]

Until an initscript is available for redis-sentinel in its package[1]
we need to roll our own. This change adds that by creating a templated
initscript with the sentinel config file and service owner and group
as template variables that get filled in.

Testing this revealed some bugs and misfeatures that are also addressed
in this change:

• apt systems want the redis and redis-sentinel processes to daemonize
  (systemd-based) rpm does not. This was not reflected in the params
  defaults, nor reflected in the sentinel configuration template. Now
  it is.
• the package_name for redis-sentinel was not being passed in as a
  parameter, instead it was just using the params.pp default for
  redis server. When [1] is resolve, it will likely mean a different
  package for redis-sentinel so we need there to be a separate parameter
  for the sentinel package that gets installed. For now the name of
  the existing package is used.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=775414

[cdent]

@antonlindstrom and @arioch as far as my testing goes on ubuntu 14.04 this seems to fix #18 as expected. It would probably be wise for other folk besides me to test it before merging it though.

As you'll see the changes cascaded a bit to requiring a few other fixes.

[cdent]

Please don't merge this yet, I left out a critical part: The initscript is not getting linked into the rc directories so the service won't launch on boot. I'll get that early tomorrow.

[cdent]

The second commit should fix the the lack of initscript, and also fixes pid file handling. I reckon this is ready for someone else to give it a try and if it passes muster, should be good to merge.

[antonlindstrom]

Looks, good, I'll do a test today.

Thank you so much, @cdent!

[antonlindstrom]

@cdent I can verify that this works as expected, thanks!

[arioch]

Thanks @antonlindstrom & @cdent!

[cdent]

\o/