Ensure cf push of the example-service-instances-api app is checked

This failed because the app manifest was hardcoded with GOVERSION=1.21
which was no longer supported by the cloudfoundry environment.
Unfortunately the test proceeded anyway and failed later in a way that
was harder to troubleshoot.

Now the feature flags tests asserts that cf push actually succeeds - or
at least cf push exits non-zero and flags a better error message
up-front.

system_tests/feature_flags/feature_flags_test.go

@@ -81,13 +81,13 @@ var _ = Describe("FeatureFlags", func() {
  siAPIUsername := "siapi"
  siAPIPassword := "siapipass"
 
- cf.Cf("push",
+ Expect(cf.Cf("push",
  "-p", os.Getenv("SI_API_PATH"),
  "-f", os.Getenv("SI_API_PATH")+"/manifest.yml",
  "--var", "app_name="+appName,
  "--var", "username="+siAPIUsername,
  "--var", "password="+siAPIPassword,
- )
+ )).To(gexec.Exit(0), `cf push example-service-instances-api app failed!`)
 
  brokerInfo = bosh.DeployBroker(
  "-feature-flag-"+uniqueID,