ENYO-1740, ENYO-1890: Dropbox, enhance & document GET

- add support for `versionTag` in the node descriptor
- add JSON schema for node descriptor
- add new `x-ares-node` HTTP header, returned by `GET`

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>