Set the Rails secret tokens using the standard AppConfig mechanism.

When the launcher runs for the first time, it will generate these
tokens automatically and store them under AppConfig[:data_directory].
This should work without modification for most people, but those
running in clustered setups will need to set the secrets in their
config.rb files and make sure they match across instances.

Updated the clustering doco to reflect all of this.
Merge pull request #379 from hudmol/secret-token

Set the Rails secret tokens using the standard AppConfig mechanism.