Fix sudo_cmd_call in tempest.common.commands

The check that proc.returncode is non-zero was never executed
because it was located after the return statement.

Moreover, subprocess.CalledProcessError may only be raised by
a process run by check_call() or check_output()
but not by communicate().

Change-Id: I5fdcdcdda85552e5855241ada34054cfda8464d5
Merge "Fix sudo_cmd_call in tempest.common.commands"