iallocator: Add node whitelist

In the future instance creations might have a lock on all nodes as was
the case until the implementation of opportunistic locking. Nodes for
which the lock is not held will be shown to the iallocator plugin as if
they were marked offline.

This patch adds a new parameter named “node_whitelist” to
“IAReqInstanceAlloc”. If set to a list, only nodes contained within are
shown as online.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>