environment support for commands run on the providers

[shadeofblue]

we'd like add an environment definition similar to what's available in docker-compose

e.g.

nodes:
  backend:
    payload: "backend"
    environment: 
      PROVIDER_ADDRESS: "http://bor.golem.network/"
      MONGO_DB_CONNECTION_STRING: "mongodb://192.168.0.2:27017"
...

it should be passed to all commands that are executed on the given node (Service)

yapapi should support this:

https://github.com/golemfactory/yapapi/blob/master/yapapi/script/__init__.py#L154

but maybe first we should perform a quick spike:

• create a simple yapapi example that passes an environment variable in a run command yapapi#1071

[approxit]

We need to decide following things:

• Do we want to implement this in dapp descriptor?
• Do we have defined syntax for this in dapp descriptor?
• What about env variables for non-vm runtimes?

[approxit]

See golemfactory/yagna#2448 for yagna implementation