Noticket: Compatibility fixes for Python 3.4:

- Fixed invocation of pexepect.spawnu to convert the passed-in
arguments explicitly into a list. Necessary since Python 3.4 can't
use the *args construct outside of assignment.
- Updated the certificate_to_pem function to return str instead of
bytes. Necessary since Python 3.4 does not support things like b"%s"
% mybytes.
- Fixed test for existence of help CLI command. Previous code was
referencing a wrong/non-existent function help (the actual name has
to be help_ in order not to shadow the built-in function).
- Updated unit test invocations that use the read_certificate
function.
- Updated tests for the read_certificate function.