Add AUTODNSSEC, implement for DNSimple (#640)

* Add AUTODNSSEC, implement for DNSimple

There are two models for DNSSEC in DNS management: either dnscontrol
manages all the DNSSEC records and has to be invoked regularly for
re-signing, or the provider manages DNSSEC automatically and dnscontrol
is not involved beyond saying "yeah, do that".

This implements the latter, as a model, and for one provider.

Potentially we could tune/configure options for DNSSEC such as
algorithm, but DNSimple don't expose that API so I haven't implemented
it.

This minimal model should be something which maps into other providers
cleanly.

* Fix missing CanAutoDNSSEC on provider

* Validation fix for master broken

This is broken in master and causing Travis in my branch to fail.  The
validation tool runs with `gofmt -s` to require "simplify", and so
rejects an ignored second bound variable to range iteration.

* Correct wire in the AUTODNSSEC validation step

providers/gcloud/gcloudProvider.go

@@ -108,7 +108,7 @@ func (g *gcloud) loadZoneInfo() error {
 // ListZones returns the list of zones (domains) in this account.
 func (g *gcloud) ListZones() ([]string, error) {
 	var zones []string
-	for i, _ := range g.zones {
+	for i := range g.zones {
 		zones = append(zones, strings.TrimSuffix(i, "."))
 	}
 	return zones, nil