python/samba/netcmd: net.change_password should be passed string

python/samba/netcmd:  net.change_password should be passed string

password param which in python2 (is str) is incorrectly encoded
before passing to net.change_password.

python2 - password is either unicode or str, if str we should
          decode to get unicode (and then pass to net.change_password).
python3 - password is either str or bytes, if bytes then decode
          (and pass as 'str' to net.change_password).
Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>