Add tray version support

[jmaupetit]

Purpose

While fetching an application tray, we need to support multiple versions of the tray.

Proposal

• update the apps tree to something like:

apps/{{ tray.name }}/{{ tray.version }}/templates/...

• add versions to active apps, e.g.:

apps:
  - name: richie
    version: 1.0.0

• collect apps given this version

Optional:

• add default version support

[jmaupetit]

Not sure this is relevant in fact. Will we deploy two different tray releases in the same project ?