Artifact Store Feature

Following functionality should be implemented:

## Artifact Store

Marathon needs to know where to store the artifacts.
The url to the aritfact store is given as command line parameter on start.
Example: --artifact_store hdfs://my.node/directory/to/store

## Artifact REST endpoint

The artifact store can be accessed and managed via http.
GET /v2/artifacts/{path} Download artifact with given path.
POST /v2/artifacts/{path} Upload artifact with given path in the artifact store. Return the complete URL of the artifact in the artifact store in the location header.
POST /v2/artifacts Upload artifact. A name is generated by the server. Return the complete URL of the artifact in the artifact store in the location header.
DELETE /v2/artifacts/{path} Delete artifact with given path.

## Artifact Cache

Marathon can use the artifact store as artifact cache.
The AppDefinition is extended with Parameter resolveUrls: []
Those urls get 
* resolved 
* stored in the artifact store
* the artifact store urls are added to uris property
as part of the deployment.