Allocate networks in the background

Setting up networks and allocating IP addresses has the potential to
take an undesirable amount of time, blocking the build of a new
instance.  We can parallelize some work by querying for this information
*while* a new instance is being provisioned in the virt driver.

This adds a wrapper around the NetworkInfo model which allows one to
query for network information in a greenthread.  When a code path is hit
that actually needs the network information, it waits for the
greenthread to finish, if it hasn't already.

Network allocation will only be async for virt drivers that accept this
NetworkInfo model.  Other drivers that expect the legacy format end up
waiting for the operation to complete before driver.spawn is called.

Implements blueprint async-network-alloc

Change-Id: I389f939b9a36bc20b3b66e98d006d694622bd963

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