Parameterize access to zk operations

service/src/main/java/org/whispersystems/textsecuregcm/configuration/ZkConfig.java

@@ -21,6 +21,10 @@ public class ZkConfig {
   @NotNull
   private byte[] serverPublic;
 
+  @JsonProperty
+  @NotNull
+  private Boolean enabled;
+
   public byte[] getServerSecret() {
     return serverSecret;
   }
@@ -28,4 +32,8 @@ public class ZkConfig {
   public byte[] getServerPublic() {
     return serverPublic;
   }
+
+  public boolean isEnabled() {
+    return enabled;
+  }
 }