Read disk template from export info

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>