Rename 'inventory.secret' directory to 'secret'

The old scheme for secret directory was created, when Ansible
inventories lived happly side by side in one shared directory. But now,
since each DebOps project has its own project directory, copying
inventory name to add '.secret' suffix to it seems unnecessary.

So now, secrets will be kept in 'secret' directory, still relative to
Ansible inventory. If you're confused, I've modified the ASCII graph in
the README to show how new directory layout looks like.

DebOps scripts also will be modified to support this change, but you
will need to rename currently existing directories yourself:

  'inventory.secret' directory becomes 'secret'
  '.encfs.inventory.secret' directory becomes '.encfs.secret'