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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  
```