[plugin/route53] Deprecate plaintext secret in Corefile for route53 plugin (#5228)

This PR deprecates plaintext secret in Corefile for route53 plugin (`aws_access_key`).
Since using environmental variables of `AWS_ACCESS_KEY_ID` and `AWS_SECRET_ACCESS_KEY`
have already been available, no other changes other than deprecation is needed.

This will avoid saving plaintext secret in Corefile which could be
of security concern.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/route53/setup.go

@@ -80,6 +80,7 @@ func setup(c *caddy.Controller) error {
 						SecretAccessKey: v[1],
 					},
 				})
+				log.Warningf("Save aws_access_key in Corefile has been deprecated, please use other authentication methods instead")
 			case "aws_endpoint":
 				if c.NextArg() {
 					endpoint = c.Val()