Updating SwiftOnFile version to release 2.0

Updated sof version and tox swift dependency to
release 2.0.
Also, made some documentation changes to the object-server
configuration sample file to point out that volume must
mounted before starting sof object-server.

Signed-off-by: Thiago da Silva <thiago@redhat.com>
https://launchpad.net/swift/juno/2.0.0/+download/swift-2.0.0.tar.gz

We usually resort to git repo as URL only when we do not have official tar.gz upstream Swift releases.