[respect-network-flag-#75464982]: [respect-network-flag-#75464982]: [respect-network-flag-#75464982]: Re-implement most of existing network pool allocation using net_fence/subnets.

Make old/ code use new implementation.

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>
Signed-off-by: Steve Powell <spowell@pivotal.io>