join/update: set dNSHostName if not set
If during a join or update an existing AD computer object does not have the dNSHostName attribute set it will be set with the current hostname. This is important for cases where the user doing the join or update only has "Validated write to service principal name" for the computer object. The validated write with fully-qualified names can only be successful if dNSHostName is set, see [MS-ADTS] section 3.1.1.5.3.1.1.4 "Validated Writes - servicePrincipalName" for details.
Resolves https://bugzilla.redhat.com/show_bug.cgi?id=1734764