docs: add azure private cluster page

[zbwright]

slight bump to @alexsomesan. this is for https://jira.prod.coreos.systems/browse/DOCS-120

[zbwright]

@alexsomesan if you're good with this, I will pass to @joshix

[zbwright]

from Alex: So there are two valid options regarding DNS there: 1) use Azure MANGED DNS (not the default one against which we already warn) or 2) Use your own externally managed DNS server about which the installer doesn’t care but which needs to resolve requests coming from the cluster nodes number 2) is an advanced use case where users already need to be familiar with managing DNS so we don’t need to elaborate very much but we need to point out that the specific DNS server needs to be configured as the default resolver for the nodes

[joshix]

@alexsomesan I am going to interpret Beth's comment above, and the eventual content in this document, in the following way:

There are three DNS options for Azure: 1) Azure "managed" DNS -- your hostnames, with Azure-region domain names 2) Azure DNS with your own domain name 3) External DNS, not Azure DNS service

Of those, only 1 and 3 can be used with a private cluster on Azure.

Please let me know if that's accurate.

[alexsomesan]

@joshix Number 1) is not supported: the default hostnames that you can resolve without a managed zone are not enough. We need a way to publish multiple-entry A records, which we can only get via 2) and 3).

So to summarize, the two options that will work are: 2) Azure DNS with your own domain name and 3) External DNS, not Azure DNS service

[joshix]

@alexsomesan Aha. Thank you!

[joshix]

@zbwright Fixed a typo and ironed out one sentence for (hopefully) clarity. If you don't hate my changes, I'll squash and merge this.