Trivial: stdin needs the 'encoding' attribute on test_setup as well

The TestHarness is created in test_setup and it attempts a call using
the neutron client. This neutron client checks for 'encoding' on stdin.
The prehook for test_setup needs to also ensure that stdin has the
encoding attribute.

Change-Id: I1eb40f010ae32cba1e6028de1b58a7189d7296d6
Merge "Trivial: stdin needs the 'encoding' attribute on test_setup as well"