fix(show-monitor): unset dashboard.id field for the restored dashboard

[vponomaryov]

Having 'dashboard.id' field be set we get following error:

Error message {"message":"Dashboard not found","status":"not-found"}

So, unset it to make Grafana understand that we create dashboard from
scratch and not update "existing' one.

PR pre-checks (self review)

• I followed KISS principle and best practices
• I didn't leave commented-out/debugging code
• I added the relevant backport labels
• New configuration option are added and documented (in sdcm/sct_config.py)
• I have added tests to cover my changes (Infrastructure only - under unit-test/ folder)
• All new and existing unit tests passed (CI)
• I have updated the Readme/doc folder accordingly (if needed)

[roydahan]

I think this PR screwed the dashboard somehow.
I'm running a 200gb-48h longevity and the name of the dashboard is: longevity-twcs-3h:
http://52.51.205.181:3000/d/J1T2FZ57za/longevity-twcs-3h-scylla-per-server-metrics-nemesis-master?orgId=1&from=now-1h&to=now

[vponomaryov]

I think this PR screwed the dashboard somehow. I'm running a 200gb-48h longevity and the name of the dashboard is: longevity-twcs-3h: http://52.51.205.181:3000/d/J1T2FZ57za/longevity-twcs-3h-scylla-per-server-metrics-nemesis-master?orgId=1&from=now-1h&to=now

This PR definitely fixed the problem that was 100% reproducible here:
https://jenkins.scylladb.com/view/QA/job/QA-tools/job/hydra-show-monitor/31/console

@roydahan do you really need ID in there?