Add endpoint for supported/available computing environments

[nuest]

api/v1/environment returns

{
  "architecture": [
    "amd64"
  ],
  "os": [
    {
      "name": "linux",
      "version": "5.4.0-48-generic"
    }
  ],
  "container_runtimes": [
    {
      "name": "Docker Engine - Community",
      "api_version": "1.40",
      "version": "19.03.13"
    }
  ],
  "erc": {
    "manifest": {
      "capture_image": "o2rproject/containerit:geospatial-0.6.0.9003",
      "base_image": "rocker/geospatial:3.6.2",
      "memory": 2147483648
    },
    "execution": {
      "memory": 4294967296
    }
  }
}

Implemented in o2r-project/o2r-muncher#123, but not yet documented here. Will catch up with that ater switchingto the Open API spec.