EVEREST-1542 - Extend upgrade tests

[tplavcic]

The upgrade test was not finished so in this PR we added/fixed:

• PXC cluster is included in the upgrade.
• Created shell script (percona-everest-ui.sh --pre-upgrade in everest-qa repo) which prepares local environment for the upgrade so it is easy to run it locally.
• Added data before upgrade and do a check after upgrade.
• Added upgrade for databases so at the moment we do Everest upgrade, operators upgrade and database upgrade.
• Unified the environment so that it is completely identical to normal UI tests for easier debugging.
• Added database deletion and Everest uninstall check.
• Added util functions getVersionServiceURL, getDbClusterAPI, updateDbClusterAPI, getDbAvailableUpgradeVersionK8S for future use.
• Tweaked resources so the whole setup can actually run on the cloud instances which we use.

NOTE:
For the util functions the naming rule will be:

• Add suffix API if the function is getting/setting something only through api calls.
• Add suffix K8S if the function is getting/setting something only in kubernetes.
• If the function doesn't have suffix it means it does some actions through UI or it does something irrelevant like some calculation or data transformation.

[fabio-silva]

@tplavcic I think we need an empty commit to trigger the checks, as there was a CI commit that won't trigger those (we'll need a token to fix this issue)