pithos: return always etag
Refs: #3554
If update md5 is deactivated Pithos does not return an etag header.Pithos in that case should return the merkle hash as etag
docs: Update list of pithos commands
Also list in a separate section the pithos-manage-accounts commands
pithos: Update pithos management commands
Refs: #3625
pithos: manage_accounts support for both sqlalchemy and sqlite