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Fix an issue about generation of secret key with invalid key size in gce
Replace /dev/random to /dev/urandom to avoid generation of secret key with invalid key size.
**What this PR does / why we need it**:
In GCE env, AESGCM encryption of secrets by default generates a secret key with /dev/random which sometime generates a key with invalid size.
This cause the cluster/kube-up.sh to fail in gce environment.
This PR replaces /dev/random with /dev/urandom to have a secret key generated consistently with right size.
**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#67091
**Special notes for your reviewer**:
**Release note**:
```release-note
cluster/gce: generate consistent key sizes in config-default.sh using /dev/urandom instead of /dev/random
```
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