LUCreateInstance: implement disk adoption mode

This new mode, valid only for the plain template disk, allows creation
of an instance based on existing logical volumes (preserving data),
rather than creation of new volumes and OS creation.

The new mode works as follows:

- instead of size, all disks passed in must have an 'adopt' key, which
  signifies the LV name to be used
- all disks must have this key, or neither should
- we check the volume existence, and from the result we fill in the
  actual size
- online (in-use) volumes are not allowed
- 'stealing' of another's instance volumes is prevented via reservation
  of the LV names
- during creation, we rename the logical volumes to the standard Ganeti
  format (based on UUID)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>