increase routecontroller resync_interval time

- the default of 30s is way too aggressive for larger components
- it takes over 90s to resync 1,000 routes
  - 900s would give us time for 9,000-10,000 routes, more than istio can
  performantly handle
- this is only for disaster recovery or cases where operators mess up
their config, so 15 minutes is still a pretty good recovery time

discovered as part of [#173163584](https://www.pivotaltracker.com/story/show/173163584)

config/routecontroller/routecontroller-configmap.yaml

@@ -10,3 +10,4 @@ metadata:
  kapp.k14s.io/num-versions: "2"
 data:
  ISTIO_GATEWAY_NAME: cf-system/istio-ingressgateway
+ RESYNC_INTERVAL: "900"