Define and configure a default path for Soledad Server BLOBs storage

Soledad has a new mechanism for storing BLOBs in filesystem. Platform needs to:

• define a default path for soledad blobs.
• include that path in backups. #8867
• monitor that path for its size. #8868
• add the path to the soledad server configuration file (blobs_path=/path/to/blobs). #8869

When thinking about the path, i suggest taking into account that lintian doesn't like stuff in /srv on debian packages. That lintian tag is non-overridable, that is, it will fail badly and we can't ignore, unless we setup another lintian vendor that defines that lintian tag as overridable. One possibility that comes to mind is /var/lib/soledad/blobs.