Expose alarm severity in Alarm Model

This commit enables us to set a severity to an alarm.
This will greatly help from auditing standpoint on which
alarms are low/moderate/critical when they were triggered.

Change-Id: I5a0d3cf4d5736983a8c8354310360d2b41892965
Implements: blueprint ceilometer-alarm-level

ceilometer/storage/sqlalchemy/migrate_repo/versions/040_add_alarm_severity.py

@@ -0,0 +1,31 @@
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+from sqlalchemy import Column
+from sqlalchemy import MetaData
+from sqlalchemy import String
+from sqlalchemy import Table
+
+
+def upgrade(migrate_engine):
+    meta = MetaData(bind=migrate_engine)
+    alarm = Table('alarm', meta, autoload=True)
+    severity = Column('severity', String(50))
+    alarm.create_column(severity)
+
+
+def downgrade(migrate_engine):
+    meta = MetaData(bind=migrate_engine)
+    alarm = Table('alarm', meta, autoload=True)
+    severity = Column('severity', String(50))
+    alarm.drop_column(severity)