auth/credentials: PY3 set_password should decode from unicode 'utf8'

auth/credentials: PY3 set_password should decode from unicode 'utf8'

set_password processes input using ParseTuple with "s" format, this
accepts string or unicode but...

Some py2 code is incorrectly using code like

   credentials.set_password(pass.encode('utf8'))

however that won't work in PY3. We should just make sure the string
retrieved from unicode passed in is encoded with 'utf8'
Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>