ipauser: Fix idempotence issue when using 'preserved'.

When trying to ensure 'state: absent' with 'preserved: yes' in ipauser,
after the first execution the playbook would fail with "user is already
present". Similar idempotence issue would happen when 'state: undelete'
was used.

This PR fixes both issues, and improve tests for the states where user
is preserved, enabled and disabled. The 'find_user' function now uses
IPA API 'user_show' instead of 'user_find' so that only the requested
user is actually returned.