slapos/recipe/redis/promise: Don't create connection pool explicitly

Because redis.Redis(...) ctor creates connection pool on initialization
and we can rely on it.

Another reason: Redis ctor (in form of StrictRedis.__init__()) has logic
how to process arguments and does selecting - either it is TCP (`host` and
`port` args), or UNIX socket (`unix_socket_path` arg):

    https://lab.nexedi.com/nexedi/slapos/blob/95dbb5b2/slapos/recipe/redis/MyRedis2410.py#L560

Since we are going to introduce unix socket support to redis recipe in
the next patch, and don't want to duplicate StrictRedis.__init__() logic
in promise code, let's refactor promise to delegate argument processing
logic to Redis.

/reviewed-by @kazuhiko  (on !27)
/cc @alain.takoudjou

slapos/recipe/redis/promise.py

@@ -8,10 +8,9 @@ def main(args):
   host = args['host']
   port = int(args['port'])
   try:
-    pool = redis.ConnectionPool(host=host, port=port, db=0)
-    r = redis.Redis(connection_pool=pool)
+    r = redis.Redis(host=host, port=port, db=0)
     r.publish("Promise-Service","SlapOS Promise")
-    pool.disconnect()
+    r.connection_pool.disconnect()
     sys.exit(0)
   except Exception, e:
     print str(e)