[scripts][execute] Make loop address an option

This makes its usage more clear and explicit.

It also fixes an issue where the loop paradigm can only be
used with a block having a database as input.

This change requires update in beat/beat.core> executors
calling these scripts.

beat/backend/python/scripts/execute.py

@@ -37,7 +37,7 @@
 """Executes a single algorithm. (%(version)s)
 
 usage:
-  %(prog)s [--debug] [--cache=<path>] <addr> <dir> [<db_addr>] [<loop_addr>]
+  %(prog)s [--debug] [--cache=<path>] [--loop=<loop_addr>] <addr> <dir> [<db_addr>]
   %(prog)s (--help)
   %(prog)s (--version)
 
@@ -47,14 +47,13 @@ arguments:
   <dir>       Directory containing all configuration required to run the user
               algorithm
   <db_addr>   Address for databases-related I/O requests
-  <loop_addr> Address for loop-related I/O requests
-
 
 options:
   -h, --help         Shows this help message and exit
   -V, --version      Shows program's version number and exit
   -d, --debug        Runs executor in debugging mode
   -c, --cache=<path> Cache prefix, otherwise defaults to '/cache'
+  --loop=loop_addr   Address for loop-related I/O requests
 
 """
 
@@ -187,10 +186,11 @@ def main():
         logger.debug("zmq client connected to db `%s'", args["<db_addr>"])
 
     loop_socket = None
-    if args["<loop_addr>"]:
+    loop_addr = args.get("--loop")
+    if loop_addr:
         loop_socket = context.socket(zmq.PAIR)
-        loop_socket.connect(args["<loop_addr>"])
-        logger.debug("zmq client connected to loop `%s'", args["<loop_addr>"])
+        loop_socket.connect(loop_addr)
+        logger.debug("zmq client connected to loop `%s'", loop_addr)
 
     # Check the dir
     if not os.path.exists(args["<dir>"]):