ensure_ddoc doesnt make sense anymore as we dont have any ddoc other
than _security, which has its own method for setting. 'ensure_security'
is explicit and is set internally when user is creating a database,
otherwise it will be False as it's only used during creation. This isn't
exposed externally (of couch module) to avoid confusion.
This confusion was making create-user-db fail to create a security ddoc
as it wasn't passing ensure_ddocs=True.

-- Resolves: #8388

As SoledadBackend is intended to be database agnostic, a new generic
document is now used instead of the old one made for CouchDB. The only
attribute that really relates to couch was couch_rev, removed on this
commit as it can be set on CouchDatabase implementation when needed.