JCLOUDS-832: Don't expose the password credentials in the toString method

apis/openstack-keystone/src/main/java/org/jclouds/openstack/keystone/v2_0/domain/PasswordCredentials.java

@@ -127,7 +127,8 @@ public class PasswordCredentials {
 
    protected ToStringHelper string() {
       return Objects.toStringHelper(this)
-            .add("username", username).add("password", password);
+            .add("username", username)
+            .add("password", password == null ? null : "*****");
    }
 
    @Override