Install Redis plugin package if needed

[bodgit]

I found the collectd-redis sub-package wasn't being installed when I included the plugin so I added the logic based on some of the other plugins. I also refactored the code to use Puppet 4 data types and tidied up the template slightly.

To keep my employers legal team happy, I have to include the following:

This contribution is provided 'as is' and without any warranty or guarantee of any kind, express or implied, including in relation to its quality, suitability for a particular purpose or non-infringement. To the extent permitted by law, in no event shall the creator of this contribution be liable for any claim, damage or other liability, whether arising in contract, tort or otherwise, arising out of or in connection with this contribution.

[oranenj]

Seems fine to me. Re-running the test suite...

[oranenj]

Our tests are finally somewhat non-broken, so I'll just merge this to get things rolling again. Seems safe enough.