Clean BDM when snapshoting volume-backed instances

When snapshoting a volume-backed instance, its block device mapping is
copied entirely into the image properties of the resulting snapshot.

This may lead to some errors when booting from that snapshot because the
fields 'id', 'insance_uuid' and 'connection_info' must be generated for
each instance instead of reusing the ones from the original instance.
For example, sharing the connection_info will cause the new instance to
try to connect to the original volume instead of creating a new one.

This patch cleans all the database-specific plus the connection_info
fields from the block device mapping when creating the snapshot.

Change-Id: I4190e9519d641c61b00b5d86c3885f2ae4fa86f3
Partial-Bug: #1246327
Merge "Clean BDM when snapshoting volume-backed instances"