install: improve CLI positional argument handling

Instead of specifying which knobs should be positional arguments in
cli.install_tool(), do it using a flag in knob definition, where the rest
of CLI configuration is.

As a side effect, the usage string for CLI tools can now be generated
automatically.

https://fedorahosted.org/freeipa/ticket/6392

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/install/server/replicainstall.py

@@ -1444,6 +1444,8 @@ class Replica(BaseServer):
     replica_file = Knob(
         str, None,
         description="a file generated by ipa-replica-prepare",
+        cli_positional=True,
+        cli_name='replica_file',
     )
 
     setup_ca = Knob(BaseServer.setup_ca)