For instances use the real instance disk template
For instances use the real instance disk template

...instead of assuming there is a single template that fits all
instances of the cluster.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

leshchevds/ganeti,andir/ganeti,ganeti/ganeti,onponomarev/ganeti,mbakke/ganeti,andir/ganeti,mbakke/ganeti,mbakke/ganeti,yiannist/ganeti,andir/ganeti,bitemyapp/ganeti,yiannist/ganeti,leshchevds/ganeti,yiannist/ganeti,grnet/snf-ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,ganeti/ganeti