Add pkg/fall for Fallthrough (#1355)

* Add pkg/fall for Fallthrough

Move this into it's own package to facilitate tests. Important bug
was fixed: make the names fully qualified.

Add fall package to hosts, reverse, etcd, and fix kubernetes and any
tests. The k8s tests are still as-is, might need a future cleanup.

plugin/reverse/README.md

@@ -15,7 +15,7 @@ response. This is only done for "address" records (PTR, A and AAAA).
 reverse NETWORK... {
     hostname TEMPLATE
     [ttl TTL]
-    [fallthrough]
+    [fallthrough [ZONES...]]
     [wildcard]
 ~~~
 
@@ -23,6 +23,9 @@ reverse NETWORK... {
 * `hostname` injects the IP and zone to a template for the hostname. Defaults to "ip-{IP}.{zone[1]}". See below for template.
 * `ttl` defaults to 60
 * `fallthrough` if zone matches and no record can be generated, pass request to the next plugin.
+  If **[ZONES...]** is omitted, then fallthrough happens for all zones for which the plugin
+  is authoritative. If specific zones are listed (for example `in-addr.arpa` and `ip6.arpa`), then only
+  queries for those zones will be subject to fallthrough.
 * `wildcard` allows matches to catch all subdomains as well.
 
 ### Template Syntax