Fix archip-openvolumes hook

This hook makes use of commit 39b2b1c in upstream Ganeti.
This commit does not export GANETI_INSTANCE_DISKX_DISK_TEMPLATE.
Instead it exports GANETI_INSTANCE_DISKX_DEV_TYPE. Use this.

Additionally this hook fails if another ext provider is used
and not archipelago which make migration of other instances
impossible. Fix this.

Last, Ganeti is about to implement instances with multiple disk
templates. Here we act per disk and skip check of
GANETI_INSTANCE_DISK_TEMPLATE.

Signed-off-by: Dimitris Aragiorgis <dimitris.aragiorgis@gmail.com>