redis: support healthcheck interval

Redis client supports health checks using PING/PONG since 3.3.0[1] but
this feature was not configurable in tooz.

This adds the new health_check_interval option, which is forwarded to
Redis client, to enable that feature.

[1] f60b2b07ca

Change-Id: Icba79be1929ec460d3cb17363e701fbee7de7d76

releasenotes/notes/redis-health-check-f5e3cb15dd3ac90c.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Redis driver now supports the ``health_check_interval`` option.

tooz/drivers/redis.py

@@ -260,6 +260,7 @@ class RedisDriver(coordination.CoordinationDriverCachedRunWatchers,
         'socket_keepalive',
         'socket_timeout',
         'socket_connect_timeout',
+        'health_check_interval',
         'ssl',
         'ssl_certfile',
         'ssl_keyfile',
@@ -298,6 +299,7 @@ class RedisDriver(coordination.CoordinationDriverCachedRunWatchers,
     CLIENT_FLOAT_ARGS = frozenset([
         'socket_timeout',
         'socket_connect_timeout',
+        'health_check_interval',
     ])
 
     #: Default socket timeout to use when none is provided.