Write public cloud docs for major cloud providers

[caseydavenport]

We need docs in this section: http://docs.projectcalico.org/v2.0/reference/public-cloud/overview

This issue covers docs for:

• AWS (Add AWS guide #258)
• GCE
• Azure

We've already got this information floating around in various other guides. We should be removing it from all of those places and keeping a "Calico on AWS", "Calico on GCE", etc style doc for each provider.

[ozdanborne]

#315 and #321 were closed, but might provide a useful base for resolving this.