scale up api to 25 instances before deploy on production

deploys take up to five minutes, during which notify-paas-autoscaler
can't scale the app. We saw 502s due to a large volume of traffic
coming in during that time, and we couldn't react cos we were
deploying.

scale up to 25 instances, the autoscaler won't be able to downscale
until after the deploy has finished.

manifest.yml.j2

@@ -1,12 +1,28 @@
 {%- set app_vars = {
-  'notify-api': {'NOTIFY_APP_NAME': 'api', 'disk_quota': '2G', 'sqlalchemy_pool_size': 20, 'routes': {
+  'notify-api': {
+    'NOTIFY_APP_NAME': 'api',
+    'disk_quota': '2G',
+    'sqlalchemy_pool_size': 20,
+    'routes': {
       'preview': ['api.notify.works'],
       'staging': ['api.staging-notify.works'],
       'production': ['api.notifications.service.gov.uk'],
     },
     'health-check-type': 'port',
+    'instances': {
+      'preview': None,
+      'staging': None,
+      'production': 25
+    },
+  },
+  'notify-api-db-migration': {
+    'NOTIFY_APP_NAME': 'api',
+    'instances': {
+      'preview': 0,
+      'staging': 0,
+      'production': 0
+    },
   },
-  'notify-api-db-migration': {'NOTIFY_APP_NAME': 'api', 'instances': 0},
 
   'notify-delivery-celery-beat': {'memory': '128M'},
   'notify-delivery-worker-jobs': {},
@@ -23,14 +39,15 @@
 } -%}
 
 {%- set app = app_vars[CF_APP] -%}
+{%- set instance_count = app.get('instances', {}).get(environment) -%}
 
 ---
 
 applications:
   - name: {{ CF_APP }}
     buildpack: python_buildpack
-    {% if 'instances' in app %}
-    instances: {{ app['instances'] }}
+    {% if instance_count is not none %}
+    instances: {{ instance_count }}
     {%- endif %}
     memory: {{ app.get('memory', '1G') }}
     disk_quota: {{ app.get('disk_quota', '1G')}}