Add log_dir param. by default and allow users to disable it

  * add log_dir param (and configure it to /var/log/glance) by default.
  * fix log_file missing setting on glance::registry
  * allow users to disable log_dir and log_file with « false » argument.

Change-Id: I0689fa15199e69cf16ff306050776d150365a09c
Closes-bug: #1282296