Invoke FingerprintManager's Reload() func during agent's SIGHUP (Fixes #14614)

[pruiz]

This PR addresses #14614, by adding the missing call to Fingerprint's Reload() method from within SIGHUP handling on agent's code.

[tgross]

LGTM!

This has missed the merge window for Nomad 1.4.0, but I'll merge it once the GA goes out (shortly) and then it'll go out in the next regular release of Nomad.

@pruiz can you add a changelog file to this PR? It'll be at .changelog/14615.txt and have the format you can see in .changelog/14749.txt. For content, probably something like:

client: Fixed a bug where fingerprinters were not reloaded when the client configuration was reloaded with SIGHUP

(If you don't get to it before I'm ready to merge this, no worries. I see the PR has been open a little bit and it'll only take me a few minutes to add it.)

[pruiz]

Added changelog file.

[tgross]

Hi @pruiz! We had to push out a quick Nomad 1.4.1 patch, but this has now been merged and will go out in the next regular patch release.

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