swift backup service checks version during restore

swift backup service checks version during restore

Modified swift backup service to check metadata version during restore
and raise an error if the backup version isn't a version that the
service knows how to handle. The versions which can be handled are
described in a dictionary mapping versions to methods which can handle
them. This will facilitate graceful handling of newer backup formats by
the swift backup service when we introduce changes.

Fixes bug: 1136174

Change-Id: Id7d05848fd448ce21f641e5cd6945477702cbe38