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re-enable nodeipam in kube-controller-manager
**What this PR does / why we need it**:
Re-enables nodeipam controller for external clouds. Also does a small refactor so that we don't need to pass in `allocateNodeCidr` into the controller.
In v1.10 we made a change (9187b343e1 (diff-f11913dc67d80d36b3d06a93f61c49cf) in https://github.com/kubernetes/kubernetes/pull/57492) where nodeipam would be disabled for any cluster that sets `--cloud-provider=external`. The original intention behind this was that the nodeipam controller is cloud specific for some clouds (only GCE at the moment) so it should be moved to the CCM (cloud controller manager). After some discussions with wg-cloud-provider it makes sense to re-enable nodeipam controller in KCM and have GCE CCM enable its own cloud-specific IPAM controller as part of [Initialize()](https://github.com/kubernetes/kubernetes/blob/master/pkg/cloudprovider/cloud.go#L33-L35). This would allow for GCE to run nodeipam in both KCM (by setting --cloud-provider=gce and --allocate-node-cidr) and in the CCM (once implemented in `Initialize()`) without disabling nodeipam in the KCM for all external clouds and avoids having to implement nodeipam in CCM.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
Re-enable nodeipam controller for external clouds.
```
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