Added support to work with redis sentinel

[ivanmp91]

• Added support to connect to a list of sentinels with -S parameter and to a specific service given with -s parameter.
• Shows in the default output the ip and port of the current master server and the total number and reachable sentinels.
• Added the checkin 'sentinels_available' which contains the number of sentinels reachable. If some of them are down the plugin returns a WARNING and shows the number of sentinels down. In case that all of them are down returns a CRIT.
• Check parameters to ensure you use "-H host" or "-S sentinel1:port,sentinel2:port" and not both at the same time.
• Updated the version of the scipt, help output and comments with the new features. NOTE: In case of use the script to connect with servers via sentinel is necessary to use at least the library version Redis-1.972 which comes with the Sentinel interface.

[willixix]

I appreciate your work and contribution. Currently busy with another project, so I'll look at this in mid June and do update of the plugin - I have my own changes and several patches in line.

[ivanmp91]

Hey! That's fine no worries! I just made some changes that are useful for me at the moment and I decided to share with you. If you see something may be improved just contact with me and I can help you in implement it. By the way congrats, you've a nice set of perl checks 😁

[eltismerino]

I guess mid-june never happened? :)

Is this project dead? It's August, one year later.