ipatests: fix ldap server url
    
    master.external_hostname was used to construct ldap url
    which caused ldappasswd utility to exit with error due to host name
    mismatch in client certificate. master.hostname should be used instead
    as this name is used to generate certificate.
    
    Fixes https://pagure.io/freeipa/issue/7844
    
    Signed-off-by: Sergey Orlov <sorlov@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>