Allow allocate of a static ip within static subnets

* Refactor subnets and fence
* Selector functionals as strategy inputs to Allocate
* allocations are stored in simple map

[#81660758]

Signed-off-by: Steve Powell <spowell@pivotal.io>
Signed-off-by: Glyn Normington <gnormington@pivotal.io>
Merge branch 'multiple-containers-per-subnet-#80635982-refactor'

* multiple-containers-per-subnet-#80635982-refactor:
  Allow allocate of a static ip within static subnets

[#80635982]

Signed-off-by: Steve Powell <spowell@pivotal.io>
Signed-off-by: Glyn Normington <gnormington@pivotal.io>