ipa[user,host]: Fail on duplucate names in the users and hosts lists

It was possible to have several entries for names with the hosts and users
lists. This resulted sometimes in errors but also unexpected changes. A new
check has been added to make sure that the names in the users and hosts
lists are unique.

New tests have been added to verify this in the existing files:
- tests/host/test_hosts.yml
- tests/user/test_users.yml