[chart/redis-ha Add pod meta label to master

[tschirmer]

Would like to see a pod label added onto the master, we can then set a pod disruption budget specifically for the current master, to prevent split brain when nodes go down or the kubernetes descheduler is run.

[DandyDeveloper]

@tschirmer Something like additionalLabels just specific to the master / slaves:

redis:
  master:
     additionalLabels: {}
  slaves:
     additionalLabels: {}