scripts/rpc: do not list commands for the main parser

The main parser will show usaged like so:
usage: rpc.py [-h] [-s SERVER_ADDR] [-p PORT] [-t TIMEOUT] [-v]
              [--verbose {DEBUG,INFO,ERROR}] [--dry_run] [--server]
              {framework_start_init,start_subsystem_init,............

The above contains very long list of all RPCs.
This is duplicates help from the subparsers, which contain
more descriptive help message.

This patch replaces the usage with:
usage: rpc.py [options]

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Ibf400987103f5571f79c57d8a9588d97f463d773
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/2125
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>

scripts/rpc.py

@@ -22,7 +22,7 @@ def print_array(a):
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser(
-        description='SPDK RPC command line interface')
+        description='SPDK RPC command line interface', usage='%(prog)s [options]')
     parser.add_argument('-s', dest='server_addr',
                         help='RPC domain socket path or IP address', default='/var/tmp/spdk.sock')
     parser.add_argument('-p', dest='port',