Allow XML/JSON output for all mp-* command

$ ./bin/nuxeoctl mp-add nuxeo-6.0-HF09 --clid ~/instance.clid --accept no --json 
{"commands": {"command": {
  "param": "nuxeo-6.0-HF09",
  "pending": "false",
  "name": "add",
  "exitCode": "0",
  "messages": {"message": {
    "level": "3",
    "time": "2016-01-14T14:52:20.479+01:00",
    "message": "Waiting for download"
  }},
  "packages": {}
}}}

$ ./bin/nuxeoctl mp-install nuxeo-6.0-HF09 --clid ~/instance.clid --accept no --json -q
Exit
{"commands": {}}

$ ./bin/nuxeoctl mp-hotfix --clid ~/instance.clid --accept no --json -q
Exit
{"commands": {}}

$ ./bin/nuxeoctl mp-install nuxeo-6.0-HF02 --clid ~/instance.clid --accept yes 
(...)
Dependency resolution:
  Installation order (2):        nuxeo-6.0-HF01-1.0.0/nuxeo-6.0-HF02-1.0.0
  Local packages to install (2): nuxeo-6.0-HF01:1.0.0, nuxeo-6.0-HF02:1.0.0

Installing nuxeo-6.0-HF01-1.0.0
(...)
Dependency resolution:
  Installation order (1):        nuxeo-6.0-HF02-1.0.0
  Unchanged packages (1):        nuxeo-6.0-HF01:1.0.0
  Local packages to install (1): nuxeo-6.0-HF02:1.0.0

Installing nuxeo-6.0-HF02-1.0.0

$ ./bin/nuxeoctl mp-install nuxeo-6.0-HF02 --clid ~/instance.clid --accept yes --json
{"commands": {"command": {
  "param": "nuxeo-6.0-HF02-1.0.0",
  "pending": "false",
  "name": "install",
  "exitCode": "0",
  "messages": {},
  "packages": {"package": {
    "licenseUrl": "http://www.gnu.org/licenses/lgpl.html",
    "visibility": "MARKETPLACE",
    "targetPlatforms": "cap-6.0",
    "productionState": "PRODUCTION_READY",
    "description": "Download and install the latest hotfix to keep your Nuxeo up-to-date\n    Changes will take effects after restart.\n\n    No DDL statements is included in this hotfix.\n\n    Main corrections provided:\n    * Fix Rebuild ES index for the whole repository\n    * Fix totalSize for Elasticsearch query with limit 0\n    * Improve Elasticsearch reindex all repository\n    * Fix random page not found error from suggestion search\n    * Fix position of copied doc in ordered folder\n    * Fix lists of integers after JSF2 migration\n    * Fix Web template parameters save\n\n\n    Complete release note is available at: \n    https://jira.nuxeo.com/secure/ReleaseNote.jspa?projectId=10011&version=16758",
    "type": "HOT_FIX",
    "supportsHotReload": "false",
    "title": "Nuxeo 6.0-HF02",
    "version": "1.0.0",
    "homePage": "http://www.nuxeo.com/",
    "dependencies": "nuxeo-6.0-HF01:1.0.0",
    "licenseType": "LGPL",
    "vendor": "Nuxeo",
    "name": "nuxeo-6.0-HF02",
    "id": "nuxeo-6.0-HF02-1.0.0",
    "state": "STARTED",
    "supported": "true",
    "validationState": "NUXEO_CERTIFIED"
  }}
}}}