Merge branch 'docs/redis-sentinel' into 'master'

Update Redis Sentinel HA password examples

Fix Redis HA documentation to make sure `redis['master_password']` is used instead of the regular `redis['password']` to configure the `gitlab-rails` instance as the first will work in all scenarios. 

cc @jnijhof

See merge request !943

doc/administration/high_availability/redis.md

@@ -492,7 +492,7 @@ which ideally should not have Redis or Sentinels on it for a HA setup.
     redis['master_name'] = 'gitlab-redis'
 
     ## The same password for Redis authentication you set up for the master node.
-    redis['password'] = 'redis-password-goes-here'
+    redis['master_password'] = 'redis-password-goes-here'
 
     ## A list of sentinels with `host` and `port`
     gitlab_rails['redis_sentinels'] = [
@@ -636,7 +636,7 @@ In `/etc/gitlab/gitlab.rb`:
 
 ```ruby
 redis['master_name'] = 'gitlab-redis'
-redis['password'] = 'redis-password-goes-here'
+redis['master_password'] = 'redis-password-goes-here'
 gitlab_rails['redis_sentinels'] = [
   {'host' => '10.0.0.1', 'port' => 26379},
   {'host' => '10.0.0.2', 'port' => 26379},