Docker app readiness polling

The Docker apps does not come online immediately after the app reports its status as "Ready" in Murano: for some period of time the app remains unavailable. This is caused by the fact that the app continues its deployment after the container has been deployed: K8s may migrate the pod or do some other housekeeping, the app itself gets downloaded from docker repo, the services are restarted etc. This may take significant - and sometimes unpredictable - intervals of time.

So, the apps have to be redesigned to include "readiness polling" at the end of the actual deployment, so that Murano does not report the completed deployment until it verifies that the app is indeed accessible. It's preferable to have some universal solution for different apps.