Support Custom Domain

[dtan4]

(Sorry for large pull request... 🙇 )

WHY

To access applications on Iruka via custom domain.

WHAT

Implement CLI commands:

• $ iruka domains
• $ iruka domain-add
• $ iruka domain-remove

and implement server logic to use custom domain.
It requires Vulcand (>= v0.8.0-beta.2) container for domain-level routing.

When application is created or new container is launched, iruka registers their information to etcd under /vulcand. Vulcand reads them and distribute requests by its Host header.

💡 This feature works well with our Sinatra application and custom domain.

⚠️ This pull request has not supported blue-green deploy and multi-containers yet...

[spesnova]

Discussion about using vulcand as iruka router

As you also know vulcand is not "production ready" yet. (see "warning": Documentation — Vulcand documentation 2.0 documentation)
So we should test iruka with vulcand in production-level environment (not production) carefully.
If vulcand is not stable or lack of features that we need in production, we have to replace vulcand to nginx or something.

Because of vulcand is much easier to deploy and configure than nginx, I think vulcand is not so bad choice for now.

[spesnova]

I think backendID is more understandable than name.
I couldn't understand what name means at first time.

[spesnova]

vulcand backend has some settings.
In the near future, it needs to enable to pass VulcandBackendAddOpts to support those setting.

[spesnova]

hmm...

Perhaps you refered to kujira (prototype version for iruka that I developed before) and its schema design about routing and domain is not best.
I think this PullRequest will be much better.
But it is hard to comment every point, so I will send you another PullRequest