make aggregation-method argument as a mandatory field

When create the gnocchi alarm, the aggregation-method is required.

Closes-Bug: #1531602
Change-Id: I65b5038053d69dc727639fbfeaa5a23108845083
(cherry picked from commit 63d629f423)

ceilometerclient/v2/shell.py

@@ -519,6 +519,7 @@ def common_alarm_gnocchi_arguments(rule_namespace, create=False):
                    help='Number of periods to evaluate over.')
         @utils.arg('--aggregation-method', metavar='<AGGREATION>',
                    dest=rule_namespace + '/aggregation_method',
+                   required=create,
                    help=('Aggregation method to use, one of: ' +
                          str(GNOCCHI_AGGREGATION) + '.'))
         @utils.arg('--comparison-operator', metavar='<OPERATOR>',