Allow disk attachment with external storage
Allow disk attachment with external storage

As external storage is not associated with a node, we have to make an
exception for that before raising an error.

Signed-off-by: Lisa Velden <f95ae155456787aef400a49db463a62bbef84ead@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

mbakke/ganeti,yiannist/ganeti,yiannist/ganeti,mbakke/ganeti,ganeti/ganeti,leshchevds/ganeti,leshchevds/ganeti,andir/ganeti,mbakke/ganeti,onponomarev/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,yiannist/ganeti,onponomarev/ganeti,ganeti/ganeti,andir/ganeti