Allow retrying network allocations separately

Introduce a new config option, 'network_allocate_retries', that allows
one to retry network allocations.  The default is 0 for no retries to
match the current behavior.

The network allocations currently get retried by a full retry of a build
via the scheduler, if those are enabled.  This patch reduces the need to
re-schedule for simple network allocation issues.

The retrying happens in the network alloc async greenthread, so for virt
drivers that support the new NetworkModel, the retrying potentially
happens in the background while the image is being downloaded, etc.

DocImpact

Change-Id: I1a5fdcccbb736fc0b1d8c0cbc3b45a8372a6aef7
Merge "Allow retrying network allocations separately"