Disable caching of Web UI login_kerberos request

IE caches login_kerberos request so SSO doesn't work after logout. This patch disables the caching.

https://fedorahosted.org/freeipa/ticket/2991

install/ui/ipa.js

@@ -323,6 +323,7 @@ IPA.get_credentials = function() {
 
     var request = {
         url: IPA.login_url,
+        cache: false,
         async: false,
         type: "GET",
         success: success_handler,