[monascaclient] "Alarm Definition Update" command may not needs "--match-by" option.

[Shinya Kawabata]

"monasca alarm-definition-update " command has "--match-by" option.
But actually it cannot be changed by this option.
According to Monasca API doc, "Once an Alarm Definition has been created, the value for match_by and any metrics in the expression cannot be changed. "
So I think this option is acceptable to remove.

reference - https://github.com/stackforge/monasca-api/blob/master/docs/monasca-api-spec.md#changing-alarm-definitions

$ monasca alarm-definition-update --help
usage: monasca alarm-definition-update [--description <DESCRIPTION>]
                                       [--alarm-actions <NOTIFICATION-ID>]
                                       [--ok-actions <NOTIFICATION-ID>]
                                       [--undetermined-actions <NOTIFICATION-ID>]
                                       [--match-by <DIMENSION_KEY1,DIMENSION_KEY2,...>]
                                       [--severity <SEVERITY>]
                                       <ALARM_DEFINITION_ID>
                                       <ALARM_DEFINITION_NAME> <EXPRESSION>
                                       <ACTIONS-ENABLED>

[Allan G (greental)]

there are reasons this fields remain, Deklan Dieterly could explain it