util/cloudenv: add Azure support & DNS IPs

And rewrite cloud detection to try to do only zero or one metadata
discovery request for all clouds, only doing a first (or second) as
confidence increases. Work remains for Windows, but a start.

And add Cloud to tailcfg.Hostinfo, which helped with testing using
"tailcfg debug hostinfo".

Updates #4983 (Linux only)
Updates #4984

Change-Id: Ib03337089122ce0cb38c34f724ba4b4812bc614e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo.go

@@ -18,6 +18,7 @@ import (
 	"go4.org/mem"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/opt"
+	"tailscale.com/util/cloudenv"
 	"tailscale.com/util/dnsname"
 	"tailscale.com/util/lineread"
 	"tailscale.com/version"
@@ -38,6 +39,7 @@ func New() *tailcfg.Hostinfo {
 		Package:     packageTypeCached(),
 		GoArch:      runtime.GOARCH,
 		DeviceModel: deviceModel(),
+		Cloud:       string(cloudenv.Get()),
 	}
 }