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.

## Automatic Artifact Resolution

Marathon can use the artifact store to automatically resolve certain artifacts.
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.
I'm in favor of supporting both, it's best for the user because it supports standard tools. The upload API is not JSON and I think ease of use trumps REST purism.