Provide nova-manage commands to refresh block device mapping connection_info

The connection_info associated with a Cinder volume attachment stashed within Nova's block device mappings can often become stale. Previously operators have had to query the database directly for an understanding of the current state of the connection_info and could only migrate or shelve the instance to force a refresh of this.

This spec aims to outline some basic and potentially backportable ``nova-manage`` commands that will allow operators to both view and refresh the connection_info of a specific block device mapping within Nova.