Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1412209

Description of problem:
When attaching local user to user-group where is already set external group from LDAP, the user disappears when clicked on Submit button. When used hammer, the user disappears after

1. foreman-rake ldap:refresh_usergroups
   No error shown in WebUI/hammer/production.log

Version-Release number of selected component (if applicable):
Satellite 6.2

How reproducible:
100%

Steps to Reproduce:
1. Set up LDAP authentication.
2. Create a user-group and in third tab "External groups" specify some external group. Submit.
3. Edit the user-group and add some local user.

Actual results:
In WebUI: local user is not saved.
With hammer: user disappears later

~~~

1. hammer user-group add-user --name 'Group Test' --user user_test
   The user has been associated
2. hammer user info --login user_test | grep -A1 'User groups'
   User groups:
   Group Test
3. sudo -u foreman /usr/sbin/foreman-rake ldap:refresh_usergroups
   Successfully decrypted field for AuthSourceLdap LDAP-test
4. hammer user info --login user_test | grep -A1 'User groups'
   User groups:

~~~

Expected results:
- User can be added to group with External group
- OR local users are not available in list of users
- OR print error that it's not possible and write it to log

Additional info:
Workaround - create user-group with external group. Than create another "composite" user-group which will contain both local users and user-group with external group.