EntityRepository: optionally limit the number of versions stored for one entity
ZooKeeperConf: make the number of stored versions available via config
GroupsResource: make root group available for versions and apps
GroupManager: group by version can give None, if version does not exist
BeanValidation/ModelValidatin: move to api level
RestResource: trait for all REST traits with common functionality. make methods for all responses
AppsResource/GroupsResource: use RestResource
Responses: superseded by RestResource
AppsResource: return the deploymentId and version for every deployment
UpgradeStrategy: remove maximumRunningFactor, since there is no use for it
protos: remove maximumRunningFactor from protos
adapt all tests to this definition