set-env should support multiple environment variables

[patrickcrocker]

Instead of this:

  - put: cloud-foundry
    params:
      commands:
      - command: set-env
        app_name: myapp-ui
        env_var_name: SOME_KEY
        env_var_value: SOME_VALUE
      - command: set-env
        app_name: myapp-ui
        env_var_name: SOME_OTHER_KEY
        env_var_value: SOME_OTHER_VALUE

let's support this:

  - put: cloud-foundry
    params:
      commands:
      - command: set-env
        app_name: myapp-ui
        environment_variables:
          SOME_KEY: SOME_VALUE
          SOME_OTHER_KEY: SOME_OTHER_VALUE

This is also consistent with how env vars are passed to zero-downtime-push (and soon push as well).

[patrickcrocker]

Implemented in v2.18.0