pwclient: Fix Python 3 encoding of received strings

pwclient: Fix Python 3 encoding of received strings

The conversion encode("utf-8") makes a byte stream which is
poorly printed with Python 3.
However this encoding is required for Popen.communicate() but must be
done after str.join() which applies to a real string.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>