Fixes HP LeftHand driver with Paramiko 1.13.0

With Paramiko 1.13.0, the method exec_command now returns Unicode.
This causes a problem when the driver tries to build the XML returned
from the LeftHand array. The XML header returned from the array defines
the encoding as encoding=UTF-8. Therefore, we must now ensure the
encoding passed to the parser is utf-8.

Change-Id: I7b504626e2d9a0ee2b62820b11f56eb136e31987
closes-bug: #1298608
(cherry picked from commit 57e8cdb9dd8e24919bc2243438a6bded53491b3e)