Automatic merge from submit-queue (batch tested with PRs 56290, 57984). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
kubeadm: more random tokens
The strategy of hex encoding a random byte array only uses the
following characters:
0123456789abcdef
Instead of the entire bootstrapping token character set:
0123456789abcdefghijklmnopqrstuvwxyz
Update the token generation to use the entire character set. This
increases the token secret from 48 bits of entropy to ~82 bits.
256^8 (1.8e+19) vs. 36^16 (7.9e+24).
Noticed this writing https://github.com/kubernetes-incubator/bootkube/pull/663
```release-note
NONE
```
/cc @mattmoyer @luxas
85e0ed797f