Fix hostname logging for Local backend

Both Formatter::Pretty and Formatter::SimpleText print the hostname
along with each command being executed. But the Local backend doesn't
have a @host, so the output looks like 'running [...] on '.

This hard-codes a null Host into Backend::Local, which is only used for
logging.
Merge pull request #141 from adammck/fix_run_locally_logging

Fix hostname logging for Local backend