Read disk template from export info

This patch changes the instance import to read the disk template
automatically from the export info, if the opcode doesn't already
specify a disk template.

To do this, we have a couple of additional changes:

- change from required parameter to optional one for disk_template
- move check for disabled file storage at ./configure time to the
  generic _CheckDiskTemplate checker
- move checks of the disk template from CheckArguments to CheckPrereq

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>