Allow log_dir to be set to false in order to disable file logging

This commit allows a user to disable logging to a directory (for
example if syslog is enabled, in order to not log twice).

Conflicts: spec/classes/heat_init_spec.rb

Closes-bug: #1282296
(cherry picked from commit 603fb207959fe6fdeca2029c29c18342fc2d6b60)
Change-Id: I814bf8199fcf57f976e95b9bbf872478694b9d3e