s4/libnet: Allow passwords containing non ascii characters to be passed

s4/libnet: Allow passwords containing non ascii characters to be passed

Although we can pass unicode to py_net_change_password unfortunately in
Python2 unicode strings are encoded with the default encoding (e.g. ascii)
 when extracting the unicode string to buffer.
In Python3 the default encoding for "s" format is utf8. Use the "es"
format instead of "s" so we can specify the encoding so behaviour is
correct in py2/py3.

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