[v0.16.0] make flannel subnetLen configurable for big clusters

[Fsero]

We had an issue in a big cluster (around 200 nodes) when rolling upgrade a change and replacing instances we ran out of subnet leases, that causes that new workers were unable to get a flannel lease and hence setting up the network and were killed due to not sending the cfn-signal in time.

This patch aims to make subnetLen configurable for big clusters, using kube-aws default flannel config the maximum is 255 nodes which means around 127 nodes and a rolling upgrade that replaces all instances (like updating the base AMI).

In the case of being unset, flanneld will keep using the default which is /24 per node.

[k8s-ci-robot]

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[codecov-io]

Codecov Report

Merging #1848 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1848      +/-   ##
==========================================
- Coverage   24.41%   24.40%   -0.01%     
==========================================
  Files          98       98              
  Lines        5116     5117       +1     
==========================================
  Hits         1249     1249              
- Misses       3727     3728       +1     
  Partials      140      140              

Impacted Files	Coverage Δ
pkg/api/cluster.go	0.00% <0.00%> (ø)

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[dominicgunn]

This is awesome, we have the same issue! Thank you.

[dominicgunn]

/lgtm