Use faultstring attribute in suds.WebFault.fault

WebFault handling fails with AttributeError if there is no element at
path '/Envelope/Body/Fault/faultstring'. This patch fixes the problem
by using faultstring attribute in WebFault.fault.

Closes-Bug: #1371547
Change-Id: Ic61b22f833dd82f4c4e2d424f320526d1bd8eacd