Support private environment variables

[justmoon]

We would add a new property to the manifest called private_environment, which defines what private environment variables are defined.

{
  "manifest": {
    "image": "sharafian/crontract",
    "environment": {
      "DEBUG": "*",
      "COUNCIL_HOSTS": "10.200.10.1:8001",
      "ILP_PLUGIN": "ilp-plugin-payment-channel-framework",
      
    },
    "private_environment": {
      "ILP_CREDENTIALS": true
    },
    "port": "5001"
  }
}

The actual values are passed outside the manifest when uploading the contract.