all: simply registering plugins (#3287)

Abstract the caddy call and make it simpler.

See #3261 for some part of the discussion.

Go from:

~~~ go
func init() {
       caddy.RegisterPlugin("any", caddy.Plugin{
               ServerType: "dns",
               Action:     setup,
       })
}
~~~

To:

~~~ go
func init() { plugin.Register("any", setup) }
~~~

This requires some external documents in coredns.io to be updated as
well; the old way still works, so it's backwards compatible.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/clouddns/setup.go

@@ -18,9 +18,8 @@ import (
 var log = clog.NewWithPlugin("clouddns")
 
 func init() {
-	caddy.RegisterPlugin("clouddns", caddy.Plugin{
-		ServerType: "dns",
-		Action: func(c *caddy.Controller) error {
+	plugin.Register("clouddns",
+		func(c *caddy.Controller) error {
 			f := func(ctx context.Context, opt option.ClientOption) (gcpDNS, error) {
 				var err error
 				var client *gcp.Service
@@ -35,7 +34,7 @@ func init() {
 			}
 			return setup(c, f)
 		},
-	})
+	)
 }
 
 func setup(c *caddy.Controller, f func(ctx context.Context, opt option.ClientOption) (gcpDNS, error)) error {