Seperate create-domains command

[captncraig]

We were previously trying to magically create domains while running push/preview. This uncovered a few problems:

1. Nameserver list was not correct after the initial pre-pass. We print a warning to run dnscontrol again, but providers may be in an odd state in the meantime.
2. Before a domain is officially created, it is hard to predict what built-in records will need to be modified or whatever. This caused problems with gclouds built-in ns records needing to be modified. It is hard to do a consistent preview with such variables.

My proposal is twofold:

1. All providers will fail on push/preview if the domain is not present on the account. This is the safest choice IMO.
2. A create-domains (name still changable) command be added for convenience. Only providers that implement a secondary interface willb e invoked to add the domain to the given account.

The idea is that a dev will add a provider to a domain, run create-domains locally, and the push changes to the build server. It is a convenience over having to manually do things in the console. Nothing more.

go run ../main.go -js test.js create-domains
Initialized 2 registrars and 5 dns service providers.
***  example5555qwerty.com
  - gcloud
Adding zone for example5555qwerty.com to gcloud account
  - r53
Adding zone for example5555qwerty.com to route 53 account

[tlimoncelli]

LGTM

Commit at will.