Without this fix, last generated private subnet address overlaps with main CIDR range provided via --network-cidr= option, which causes error.
For example before change, with --network-cidr=10.0.0.0/22, the list of subnets generated by the code was:
```
I1117 07:34:24.720380 47964 cluster.go:503] Assigned CIDR 10.0.1.128/25 to zone us-east-1c
I1117 07:34:24.720397 47964 cluster.go:514] Assigned Private CIDR 10.0.3.0/25 to zone us-east-1c
I1117 07:34:24.720404 47964 cluster.go:503] Assigned CIDR 10.0.2.0/25 to zone us-east-1d
I1117 07:34:24.720409 47964 cluster.go:514] Assigned Private CIDR 10.0.3.128/25 to zone us-east-1d
I1117 07:34:24.720415 47964 cluster.go:503] Assigned CIDR 10.0.2.128/25 to zone us-east-1e
I1117 07:34:24.720420 47964 cluster.go:514] Assigned Private CIDR 10.0.4.0/25 to zone us-east-1e
```
The last CIDR 10.0.4.0/25 is beyond 10.0.0.0/22 boundaries, which causes the error:
```
W1117 07:39:29.240474 48009 executor.go:100] error running task "subnet/private-us-east-1e.kubpriv.pink-ptdevcloud.com": error creating subnet: InvalidSubnet.Range: The CIDR '10.0.4.0/25' is invalid.
status code: 400, request id: b195c64b-0a35-413c-b6ec-d7ee40d49adb
```
With a code fix, subnets get generated in a correct way:
```
I1118 07:22:31.466899 55710 cluster.go:503] Assigned CIDR 10.0.1.0/25 to zone us-east-1c
I1118 07:22:31.466908 55710 cluster.go:514] Assigned Private CIDR 10.0.2.128/25 to zone us-east-1c
I1118 07:22:31.466913 55710 cluster.go:503] Assigned CIDR 10.0.1.128/25 to zone us-east-1d
I1118 07:22:31.466917 55710 cluster.go:514] Assigned Private CIDR 10.0.3.0/25 to zone us-east-1d
I1118 07:22:31.466922 55710 cluster.go:503] Assigned CIDR 10.0.2.0/25 to zone us-east-1e
I1118 07:22:31.466925 55710 cluster.go:514] Assigned Private CIDR 10.0.3.128/25 to zone us-east-1e
```
This is a breaking change for people using the API (sorry), but is
hopefully a simple search and replace:
"k8s.io/kops/upup/pkg/api"
-> api "k8s.io/kops/pkg/apis/kops"
"k8s.io/kops/upup/pkg/api/registry"
-> "k8s.io/kops/pkg/apis/kops/registry"
This is the "correct" place for it in the k8s API infrastructure - we
are working towards a versioned API here.