Abstracting the guestagent manager implementations into pluggable packages

To increase the modularity and abstraction in trove, the idea is to separate the manager implementations for the specific service-types into pluggable packages which are loaded dynamically from within trove based on the service-type. The guiding contract however still stays within trove. The implementations in the pluggable packages implement/extend this contract further per their own specific strategies. This will help trove stay generic even with introduction of new service types.