Add linux NodeSelector to generic NodeConfig example

[rzetelskik]

Description of your changes: NodeConfig example for generic environments added in #1970 is missing kubernetes.io/os: linux node selector. This PR fixes it.

Which issue is resolved by this Pull Request:
Resolves #

/kind feature
/priority important-longterm
/cc zimnx
/assign zimnx

[zimnx]

/lgtm

[tnozicka]

/approve
/lgtm

[scylla-operator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rzetelskik, tnozicka, zimnx

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[rzetelskik]

@rzetelskik: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name Commit Details Required Rerun command
ci/prow/e2e-gke-parallel-clusterip 60fdcca link true /test e2e-gke-parallel-clusterip
Full PR test history. Your PR dashboard.

#1028 (comment)
shard-awareness flake, unrelated
/retest

[rzetelskik]

@rzetelskik: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name Commit Details Required Rerun command
ci/prow/e2e-gke-parallel-clusterip 60fdcca link unknown /test e2e-gke-parallel-clusterip
Full PR test history. Your PR dashboard.

#1233 (comment)
unrelated flake
/retest