Make `queue` volume optional

[gbartolini]

Right now, a Server resource must define a queue volume in case tier 1 is available.

For better usability, evaluate:

1. If this is needed in the first place
2. If yes, if it can be made optional with a fallback being a folder inside the tier1.data volume, by providing a way to:
   a. add it on an existing server with a rolling restart (which moves the folder to the new volume)
   b. remove it from an existing server (with a similar behaviour as the previous entry)