Use case: I'm running refstack-client in a tox job, which always appears
to succeed, even when the test fails.
Currently, unless it throws a backtrace on it's own, the refstack-client
script always returns successfully, even when the tempest job it's
running fails.
This change simply passes non-zero return codes back through as the
refstack-client's return code.
Only the test() method was affected here, because it's the only
externally callable function that uses subprocess. Other functions can
now return non-zero return codes as well.
Change-Id: I0ce786babcbee47da1c038a3478bcd4de0dff16e
1b7f413b60