Update bind docs and Getting Started

[captncraig]

Relaxes a few things to make getting started easier.

1. Don't strictly need a creds.json. I it is not there, or a provider not in it, it is up to the provider to complain it does no thave the config it needs.
2. "bind" is no longer a default skipped provider. We need to update our own scripts to work around this.
3. Getting started now runs a simple bind domain.

[captncraig]

@tlimoncelli I'm not sure if there is anything we can put for these defaults that makes more sense, but this still strikes me as odd to have DEFAULT_NOT_SET.

[captncraig]

In latest change, can make provider "non-default" in creds.json:

{
"bind":{
  "_exclude_from_defaults": "true"
}
}