IAllocator changes to work with shared storage

Make cmdlib.IAllocator shared-storage-aware. IAllocator requires secondary
nodes only on DTS_NET_MIRROR disk templates and requires no secondaries for
DTS_EXT_MIRROR templates.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>