Add the ability to create subnodes

An image can specify that it should be created with a number of
subnodes.  That number of nodes of the same image type will also
be created and associated with each primary node of that image
type.

Adjust the allocator to accomodate the expected loss of capacity
associated with subnodes.

If a node has subnodes, wait until they are all in the ready state
before declaring a node ready.

Change-Id: Ia4b315b1ed2999da96aab60c5c02ea2ce7667494