hipchat export - allow user to set the export working directory, default to the NFS share in Data Center

The export temp files go into /file_store/scissortail by default, this is configured in /hipchat-scm/chef-repo/cookbooks/scissortail/attributes/default.rb:

default[:scissortail][:working_dir] = "/file_store/scissortail"

• The user should be able to override this parameter from the command line.
• In Data Center, this parameter should default to /file_store/shared/scissortail to leverage the NFS share disk capacity.