Initial version of Kubernetes Cluster Monitor

This PR contains the following stuffs

* KubernetesClusterMonitor which is periodically performing min check
* a constant KUBERNETES_MIN_REPLICAS in StratosConstants.java
* changes in Autoscaler to add/remove kubernetes cluster monitors on cluster added/removed/maintenance event
* included isKubernetesCluster attribute in ClusterRemovedEvent class
* included isKubernetesCluster attribute in ClusterContext class

(we need above 2 attributes to decide which type of monitor to process on cluster removed/maintenance event)