Support redis sentinels and add master property

[modax]

Hello,

this PR adds support for redis sentinels (since sensu 0.23) and adds master property for redis sentinels config (undocumented sensu 0.23 feature). It is somewhat inspired on PR #496 but:

1. No sentinel unrelated changes in the code.
2. Code should be cleaner (I have kept the same type / provider for redis config with sentinels)
3. host, port, master, sentinels properties are properly removed from redis.json once they are removed from puppet config or when "connection type" changes.
4. New tests for sentinels.
5. Support for 'master' property Search Doesn't Appear to Fully Work sensu-docs#380

It would be great if you could release a new version with this PR. Thank you.

[modax]

Quick note, Puppet 4 tests are failing due to Puppet 4.5 (previous master branch was built with Puppet 4.4) which is unrelated to this PR.

[RiRa12621]

+1

[jaxxstorm]

@jamtur01 @jlambert121 any chance we can get this merged?

If you guys don't have time to manage the module, any chance you add another participant so they can help manage the module?

/cc @portertech

[jaxxstorm]

closes #514 #499