settings.production: Allow the use of azure backend if defined

This:

• adapts the deposit to allow (optionally) the use of remote azure backend (through the settings.production) [1]
• adapts accordingly the deposit private api part which checks or reads the stored archives

Refs. swh/infra/sysadm-environment#5129 (closed)

[1] Otherwise, that needs overcomplicated indirections in the chart deployment