Improve ceilometer alarm enabled input parameter validation

strict the input parameter of enabled in the command :
ceilometer alarm-threshold-update <alarm-id> --enabled {True|False}

This commit will improve the enabled value validation, if the input
value is invalid such as "abde" "enabled", error info will be returned.

Closes-Bug: #1531042
Change-Id: Ic553bc9ffa05a84575fc5626416c79fc84687bbe

ceilometerclient/v2/shell.py

@@ -464,7 +464,8 @@ def common_alarm_arguments(create=False):
         @utils.arg('--severity', metavar='<SEVERITY>',
                    help='Severity of the alarm, one of: '
                         + str(ALARM_SEVERITY))
-        @utils.arg('--enabled', type=strutils.bool_from_string,
+        @utils.arg('--enabled',
+                   type=lambda v: strutils.bool_from_string(v, True),
                    metavar='{True|False}',
                    help='True if alarm evaluation/actioning is enabled.')
         @utils.arg('--alarm-action', dest='alarm_actions',