Backport of helper: guard against negative inputs into random stagger into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #14497 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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This PR modifies RandomStagger to protect against negative input
values. If the given interval is negative, the value returned will
be somewhere in the stratosphere. Instead, treat negative inputs
like zero, returning zero.

Before:

=== RUN   TestCluster_RandomStagger
    interface.go:24: 
        cluster_test.go:27: expected 2409629h13m43.082153551s <= 1s
--- FAIL: TestCluster_RandomStagger (0.00s)

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[shoenig]

In 1.2.X this function is defined in Consul

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