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 not to login twice.

Conflicts: spec/classes/neutron_server_spec.rb

(cherry picked from commit 2efcff08d3aac7dcca9a6e1fa228a63db36de06e)
Change-Id: I0d201b8892cdaea86714b39f7716e763a40a2adc
Closes-bug: #1282296
Merge "Allow log_dir to be set to false in order to disable file logging" into stable/havana