GetNameservers is inconsistent across providers (#655)

* Warn if GetNameservers returns FQDN+dot strings
* Simplify logic that covers for the inconsistency
* Fix azuredns, gcloud, bind, route53
* Clean up cloudflare, digitalocean, dnsimple, gandi_v5, namedotcom

models/tdwarn.go

@@ -0,0 +1,13 @@
+package models
+
+import "fmt"
+
+var dotwarned = map[string]bool{}
+
+func WarnNameserverDot(p, w string) {
+	if dotwarned[p] {
+		return
+	}
+	fmt.Printf("Warning: provider %s could be improved. See https://github.com/StackExchange/dnscontrol/issues/491 (%s)\n", p, w)
+	dotwarned[p] = true
+}