Migration for detached volumes with no snaps.

Implementation of volume migration for detached volumes with no
snapshots. Migration is initiated by an admin API. The scheduler
confirms that the specified destination host can accept the volume.
The source driver is given the opportunity to migrate the volume on
their own. Otherwise, a new volume is created on the destination, both
volumes are attached, the data is copied over, the volumes are
detached, the source is deleted, and the destination is renamed. In
the database, the destination volume's attributes are copied to the
source so that the volume-id remains unchanged, and the destination
volume row is deleted.

DocImpact

Implements: bp volume-migration
Change-Id: Ib6fcf27051f45e60aa3ba5f599e88c1421db753e
Merge "Migration for detached volumes with no snaps."