[check-redis] add replication subcommand

[yuuki]

add replication subcommand insteam of slave
because it seems that the behavior that slave command returns OK if role is master is not intuitive in the PR(#259).

[y_uuki@redis01 ~]$ redis-cli info | grep role
role:master
[y_uuki@redis01 ~]$ ./check-redis replication
Redis Replication UNKNOWN: couldn't get master_link_status
[y_uuki@redis01 ~]$ ./check-redis replication --skip-master
Redis Replication OK: role is master

[y_uuki@redis002 ~]$ redis-cli info | grep role
role:slave
[y_uuki@redis002 ~]$ ./check-redis replication
Redis Replication OK: master_link_status: up

[astj]

When role is "sentinel" and --skip-master is specified, this implementation reports as ok. but imo in that case it should return unknown?
https://redis.io/commands/role

[astj]

Oh sorry, role in info cannot be sentinel? If so, it's no problem.
https://redis.io/commands/INFO

[yuuki]

yes, role in info is 'master' or 'slave' or empty. A redis sentinel process returns role replication as an empty reply.