adtrust: make sure that runtime hostname result is consistent with the configuration
    
    FreeIPA's `ipasam` module to Samba uses gethostname() call to identify
    own server's host name. This value is then used in multiple places,
    including construction of cifs/host.name principal. `ipasam` module
    always uses GSSAPI authentication when talking to LDAP, so Kerberos
    keys must be available in the /etc/samba/samba.keytab. However, if
    the principal was created using non-FQDN name but system reports
    FQDN name, `ipasam` will fail to acquire Kerberos credentials.
    Same with FQDN principal and non-FQDN hostname.
    
    Also host name and principal name must have the same case.
    
    Report an error when configuring ADTrust instance with inconsistent
    runtime hostname and configuration. This prevents errors like this:
    
        [20/21]: starting CIFS services
        ipa         : CRITICAL CIFS services failed to start
    
        where samba logs have this:
    
        [2017/03/20 06:34:27.385307,  0] ipa_sam.c:4193(bind_callback_cleanup)
          kerberos error: code=-1765328203, message=Keytab contains no suitable keys for cifs/ipatrust@EXAMPLE.COM
        [2017/03/20 06:34:27.385476,  1] ../source3/lib/smbldap.c:1206(get_cached_ldap_connect)
          Connection to LDAP server failed for the 16 try!
    
    Fixes https://pagure.io/freeipa/issue/6786
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>