store salt to regenerate box with same salt.php config file

[ceeram]

Currently fresh installs will generate unique salt and store in console/config/salt.php
salt is used for hashing passwords as well, so being able to reuse the salt when recreating the box is especially important once console will depend on a database with hashed passwords.

[bravo-kernel]

Awesome preparation, please keep in mind as discussed:

• provision upload to /home/vagrant/.cakebox/last-known-salt
• we might not use a database but flatfile or similar
• salt will be included in BackupShell