Always decode console output with stdout.encoding

Previously, euca-get-console-output always decoded console output as
UTF-8 unless instructed otherwise, which broke the final print statement
if sys.stdout.encoding happens to be ascii.  Instead, we now always
attempt to decode using sys.stdout.encoding and replace non-decodable
characters with ? characters.  To change this encoding, set LC_CTYPE in
the environment.

This also drops the recently-added --encoding option.

Fixes TOOLS-349