Add encrypt.keyStore.secret property

So that the keystore and the key can have independent secrets
(the "secret" is the key, and the "password" is the store).

Fixes gh-98

docs/src/main/asciidoc/spring-cloud-config.adoc

@@ -287,8 +287,9 @@ your `application.yml` for the Config Server:
 encrypt:
   keyStore:
     location: classpath:/server.jks
-    alias: mytestkey
     password: letmein
+    alias: mytestkey
+    secret: changeme
 ----
 
 === Embedding the Config Server