pytest/testlib: Fix related to removing kerberos database
    
    Stop krb5kdc and kadmin services. Earlier we were
    not stopping kadmin services. The current code use
    loop to stop the services
    
    Based on review comments, used tuple to define krb5kdc and kadmin
    
    Signed-off-by: Niranjan M.R <mrniranjan@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>