domainlevel-get: fix various issues when running as non-admin
Use proper filter that is caught up by the ACI for 'permission:System:
Read Domain Level' to allow any authenticated user to see the domain
level.
If the server doesn't have domain level set, callers in replica
installer expect errors.NotFound but never get it.
Return the right exception here and change the other caller to follow
the same convention.
Inability to retrieve ipaDomainLevel attribute due to a filter mismatch
casues ipa-replica-install to fail if run as a replica host principal.
Use DOMAIN_LEVEL_0 constant instead of 0 as used by the rest of the code.
Fixes: https://pagure.io/freeipa/issue/7876
Reviewed-By: Christian Heimes <cheimes@redhat.com>