Fixed credentials enforcing, updated readme

Change-Id: Ibc90ed7611fbe70f343abb297d1cc8fb6a947021
diff --git a/_states/jenkins_credential.py b/_states/jenkins_credential.py
index 22d3e72..67be9a0 100644
--- a/_states/jenkins_credential.py
+++ b/_states/jenkins_credential.py
@@ -52,7 +52,7 @@
     }
     // remove credentails with same if before created new one, if exists
     def existingCreds = com.cloudbees.plugins.credentials.CredentialsProvider.lookupCredentials(
-        com.cloudbees.plugins.credentials.common.StandardUsernameCredentials.class,
+        com.cloudbees.plugins.credentials.common.StandardCredentials.class,
         Jenkins.instance).find{it -> it.id.equals("${name}")}
     if(existingCreds){
         store.removeCredentials(domain, existingCreds)