python/samba: PY3 don't call str for bytes (or str)

python/samba: PY3 don't call str for bytes (or str)

Note: Fix needed also for gpo.apply

minPwdAge, maxPwdAge, minPwdLength & set_pwdProperties all
have a line like

value = str(value).encode('utf8')

this is a generic type statement I guess to convert int, float etc
to utf8 encoded bytes representing the string value for those.

This worked fine in PY2 but in py3 some routine already are passing
bytes into these methods, in these cases e.g. b'200' will get converted
to "b'200'", this change only performs the conversion above for non
bytes (or str) types by replacing the above with

        if not isinstance(value, binary_type):
            value = str(value).encode('utf8')

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>