[v1.15] Use a portable method to get systemd unit statuses

[rzetelskik]

Description of your changes: Backport of #2211

Which issue is resolved by this Pull Request:
Resolves #

/kind bug
/priority critical-urgent
/cc

/hold
for @grzywin to verify #2211 fixes #2180

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In response to this:

Description of your changes: Backport of #2211

Which issue is resolved by this Pull Request:
Resolves #

/kind bug
/priority critical-urgent
/cc

/hold
for @grzywin to verify #2211 fixes #2180

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[tnozicka]

/lgtm

[scylla-operator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[rzetelskik]

/hold cancel

[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 9a13009 link true /test e2e-gke-parallel
Full PR test history. Your PR dashboard.

#2185 (comment)
known manager flake
/retest