Introduce net_fence/subnets package

[respect-network-flag-#75464982]

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>
Added flags parsing for net_fence

First step to modularising container fences

[respect-network-flag-#75464982]

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>
Introduces new static allocation of container/30 subnets.

* Commence use of net_fence in new structure
* Adapt old to use new net_fence
* Added doc to pool.go

Previously NETWORK parameter was ignored. All containers allocated dynamically
in /30 subnets from the DefaultNetworkPool.
Now the NETWORK parameter can specify a /30 subnet *not* in the DefaultNetworkPool
and the container is explicitly allocated with this subnet. If not specified
it is dynamically allocated as before.

[respect-network-flag-#75464982]:

Signed-off-by: Julian Friedman <julz.friedman@uk.ibm.com>
Merge branch 'respect-network-flag-#75464982'

* respect-network-flag-#75464982:
  Introduces new static allocation of container/30 subnets.
  Introduce net_fence/subnets package

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