Add WHOIS disclaimer text to ConfigModule

This fixes #23 for @parsoj by allowing a custom disclaimer to be
specified via dependency injection modules.

By making the disclaimer part of the dependency injection graph, it can
come from anywhere.

For example, if I was Donuts, I would have my own repository. I'd use an
external http_archive() repository for Domain Registry. Then I would
write my own Dagger @Component for each App Engine module. My Component
would have a list of Dagger Modules, which I copied from the Domain
Registry version. Then I would swap out ConfigModule with my own
DonutsConfigModule, which provides the same values.

So long as a method exists that @Provides @Config("whoisRegistry"), and
the module containing it is listed in the @Component, the dependency
injection graph becomes valid and complete for the whois package
(provided other dependencies are met.)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128082921

java/google/registry/config/RegistryConfig.java

@@ -243,4 +243,6 @@ public interface RegistryConfig {
    * the logs from filling up with unnecessarily failures.
    */
   public Duration getAsyncFlowFailureBackoff();
+
+  // XXX: Please consider using ConfigModule instead of adding new methods to this file.
 }