logging_manager: Implement LoggingFile.isatty() for completeness, return False


yum-cli uses sys.stdout.isatty() in various places, so it will fail when
called from autotest since LoggingFile.isatty() does not exist.
Return False since LoggingFile is not emulating a tty.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>