PMM Upgrade from old 2.x versions to 2.2.0 shows Grafana Update Error and needs a manual refresh

On trying to Upgrade from PMM 2.x (old version) to PMM 2.2.0 when the users click on `OK`  button on successful update popup as soon as it appears, we have this race condition, and on reload we see a Grafana update error. 

This error is caused because "pmm-update / pmm-managed part does not expect that Grafana will be restarted by dashboards upgrade script"

Current workaround to this problem is to click on Reload button or refresh the page and the update works fine. 

 
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To test it:

• replace version inside /srv/grafana/PERCONA_DASHBOARDS_VERSION with some previous like 2.0.0;
• run supervisorctl tail -f dashboard-upgrade;
• alt+click on update button to force update process;
• log window should stay on screen for a few seconds and wait for dashboard-upgrade to finish;
• check dashboard-upgrade logs.