client: ignore override errors in command overrides

This fixes API initialization errors when the remote server does not have
the overriden command.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaclient/plugins/server.py

@@ -9,7 +9,7 @@ from ipalib.plugable import Registry
 register = Registry()
 
 
-@register(override=True)
+@register(override=True, no_fail=True)
 class server_del(MethodOverride):
     def interactive_prompt_callback(self, kw):
         self.api.Backend.textui.print_plain(