Do not remove the old masters when setting the attribute fails
If the setting of server attribute fails (e.g. due to master not having
the associated role enabled) the error would pop up *after* the old
values were cleared from LDAP. Fix this behavior so that all checks are
performed before manipulating any data.
https://pagure.io/freeipa/issue/7029
Reviewed-By: Martin Basti <mbasti@redhat.com>