ResolveArtifactsActor: download and store artifact
marathon.proto: store resolveUrls
AppDefinition: add resolveUrls
AppUpdate/AppUpdateBuilder: add resolveUrls
PathFun: path related functions
ModelValidation: check if resolveUrls can be resolved
Group: updateApp function to change app definitions
GroupManager: resolveUrls in target group
DeploymentActor: handle ResolveArtifacts deployment action
DeploymentManager/MarathonModule/MarathonSchedulerActor/GroupManagerTest: add StorageProvider
DeploymentPlan: create ResolveArtifacts actions
MarathonModule: HDFS: if no path is given, take root
NoStorageProvider: remove NoStorageItem and throw an exception in provider.