iallocator: use lookup by disk template

iallocator: use lookup by disk template

So far, the iallocator requested storage information
about all enabled disk templates but discarded all but
the LVM information, even if it was allocating space
for a non-LVM instance. With this patch, it now only
asks for the one that is relevant for
the allocation request.

This has the following advantages:
- less load in the RPC call
- meaningful storage information is used for non-LVM
  instance and thus better allocation

Note that this so far works only for instance allocation.
The code introduces some FIXMEs which will be resolved
when utilizing the improvements in other iallocator
requests as well.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>