Expose more info about the available environments from the REST API

Right now we get something like this:

[
  {
    "name": "Cxx backend",
    "queues": {
      "queue": {
        "memory_limit": 1985,
        "nb_slots": 0,
        "max_slots_per_user": 8,
        "nb_cores_per_slot": 1,
        "time_limit": 1440
      }
    },
    "accessibility": "public",
    "languages": [
      "cxx"
    ],
    "version": "1.1.0",
    "short_description": "C++ backend"
  },
  {
    "name": "Python 2.7",
    "queues": {
      "queue": {
        "memory_limit": 1985,
        "nb_slots": 0,
        "max_slots_per_user": 8,
        "nb_cores_per_slot": 1,
        "time_limit": 1440
      }
    },
    "accessibility": "public",
    "languages": [
      "python"
    ],
    "version": "1.2.0",
    "short_description": "Local python interpreter"
  }
]

We need to add a "modules" field to the environments that lists packages & versions that are available. It would look something like this:

[
...
  {
    "name": "Python 2.7",
    ...
    "packages": {
        "alabaster": "0.7.10",
        "Babel": "2.4.0",
        ...
    },
    "version": "1.2.0",
    "short_description": "Local python interpreter"
  }
]

This is needed for the June 2018 deliverable!