[Deploy] environment variables

[petermikitsh]

Deployments aren't too useful if you can't attach environment variables to them.

Acceptance criteria for this issue:

The snow deploy command will recognize the following values in now.json:

    "env": {
      "MY_KEY": "this is the value",
      "SECRET": "@my-secret-name"
    }

Keys will have a 1:1 map with environment variable names.

Values that begin with @ will reference a secret by the same name, e.g.,

@my-secret-name refers to secret my-secret-name.