Make gitlab log directories mode configurable

Log collection services like splunk and Sumo Logic often need read access to GitLab log files. These services do not run as root, so the default group permissions for log directories (0700) do not allow access for them. Using logging['log_group'] which sets the mode to 0750 helps, but is not a complete solution since new files created in the directory are have group set to the original owner so cannot be read.

We should add a new logging option logging['log_dir_mode'] which (in conjunction with logging['log_group']) can then be set to 02750 to solve this access issue. Alternatively, could just use the mode 02750 instead of 0750 if the logging['log_group'] is set.