Fix ipa_command_invalid_param_choices for IPA 4.6 (RHEL-7)

- krbprincipalauthind in host_add does not have choices defined
- krbprincipalauthind in service_add does not have choices defined

api.Command[command].params[name].cli_metavar returns "STR" and
ast.literal_eval failes with a ValueError "malformed string".

There is no way to verify that the given values are valid or not in
this case. The check is done later on while applying the change
with host_add, host_mod, service_add and service_mod.