Change IntegerField to ChoiceField for notification period
Change IntegerField to ChoiceField for notification period

Story: 2001506
Task: 6297

Change-Id: I937507e9a65dbf4cd3962c0910180464378aed00

stackforge/monasca-ui,stackforge/monasca-ui,openstack/monasca-ui,openstack/monasca-ui,openstack/monasca-ui,openstack/monasca-ui,stackforge/monasca-ui,stackforge/monasca-ui