Release locks before wiping disks during instance creation

Ganeti 2.3 introduced an optional feature to overwrite an instance's
disks on creation. Unfortunately the code kept all locks while doing the
wipe, slowing down the creation of multiple instances in parallel.

This patch changes the code to wipe the disks only after releasing the
locks.

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