Set prometheus server's deployment strategy to recreate

During deployment of 2i2c-org#720,
sometimes CI would fail because the prometheus pod would be
stuck in 'ContainerCreating', as the old pod was holding on to
the persistent disk the new pod needs to start. This was temporarily
fixed by deleting the prometheus pod, but this tells kubernetes
to delete the old pod properly first before starting the new one.

Ref 2i2c-org#720 (comment)

support/values.yaml

@@ -24,6 +24,10 @@ prometheus:
   pushgateway:
     enabled: false
   server:
+    strategy:
+      # We have a persistent disk attached, so the default (RollingUpdate)
+      # can sometimes get 'stuck' and require pods to be manually deleted.
+      type: Recreate
     resources:
       # Without this, prometheus can easily starve users
       requests: