Fix: Adding alarm arn to describe alarms response (#3409)

* Fix: adding alarm arn to describe alarms response

* Fix:Delete subscriptions on delete topic

* modified tests

Co-authored-by: usmankb <usman@krazybee.com>

tests/test_cloudwatch/test_cloudwatch.py

@@ -3,6 +3,8 @@ from boto.ec2.cloudwatch.alarm import MetricAlarm
 from boto.s3.key import Key
 from datetime import datetime
 import sure  # noqa
+from moto.cloudwatch.utils import make_arn_for_alarm
+from moto.core import ACCOUNT_ID
 
 from moto import mock_cloudwatch_deprecated, mock_s3_deprecated
 
@@ -51,6 +53,7 @@ def test_create_alarm():
     list(alarm.ok_actions).should.equal(["arn:ok"])
     list(alarm.insufficient_data_actions).should.equal(["arn:insufficient"])
     alarm.unit.should.equal("Seconds")
+    assert "tester" in alarm.alarm_arn
 
 
 @mock_cloudwatch_deprecated