Introduce a constant for the copyable disk templates

This list contains the disk templates suitable for moving
an instance by copying the files. A requirement is that
they're not accessed externally or shared between nodes;
in particular, sharedfile is not suitable.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>