wip - move network pool in to fence

 - technical debt: duplication of code between subnets and ip_pool
 - need to consider: should we vend Network objects instead of
 net.IPNets?
 - remaining work: add AllocateStatically and use it to enable static
 allocation of networks in createContainer
 - remaining work: move over Remove(..) methods so that resuming works
 - remaining work: configure with flags.go

[respect-network-flag-#75464982]

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>