Fix ssh.Client retval and deadlock danger LP#1038561
Fix ssh.Client retval and deadlock danger LP#1038561

Returning tuples is not handy for further checking of error states.
Using exception with exit_status and stderr output as its args instead.

There was a possibility to deadlock when reading only stdout. Solved by
using `select`.

Change-Id: Iead3540a7bd0bfce3ad092620b86ef11b9f0b622

cneill/tempest-lib,sorenh/tempest-lib,JioCloud/tempest-lib,afaheem88/tempest-lib,varunarya10/tempest-lib,mtreinish/tempest-lib