Change default hostname in rpc_tracker

This change fix problem with version of IP protocol on MacOS.
Previous the rpc_tracker and query_rpc_tracker were not able connect to
each other with default hostnames.
The root cause was in method `socket.getaddrinfo`. In rpc_tracker the
default hostname was "0.0.0.0" and `getaddrinfo` returned IPv4 type. In
query_rpc_tracker the default hastname is "localhost" and `getaddrinfo`
on MacOS returns IPv6 type. Note: on Linux both have IPv4 type.
These tools worked by different protocols and this is why
query_rpc_tracker wasn't able connect to rpc_tracker.

Now the default hostnames are the same. So it works fine on MacOS.

python/tvm/exec/rpc_tracker.py

@@ -33,7 +33,7 @@ def main(args):
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser()
-    parser.add_argument("--host", type=str, default="0.0.0.0", help="the hostname of the tracker")
+    parser.add_argument("--host", type=str, default="localhost", help="the hostname of the tracker")
     parser.add_argument("--port", type=int, default=9190, help="The port of the RPC")
     parser.add_argument("--port-end", type=int, default=9199, help="The end search port of the RPC")
     parser.add_argument(