[WIP] feat: add processor and processor group comments

[tlvince]

Adds comments to the processor and processor group resources.

WIP terraform plan picks up these changes, but after terraform apply, I'm not seeing them in NiFi. Any ideas?

Closes #22.

[egorpushkin]

LGTM

[tlvince]

@egorpushkin could you give it a try? I'd expect something like the following to (eventually) work:

resource "nifi_process_group" "local_flow" {
  component {
    parent_group_id = "${var.nifi_root_process_group_id}"
    name = "local_flow"
    comments = "hello world"

    position {
      x = 0
      y = 0
    }
  }
}