Merge pull request juju#11827 from achilleasa/2.7-backport-cleanup-sc…

…ript-changes

juju#11827

## Description of change

This PR back-ports a set of improvements to the manually provisioned machine clean up script. More specifically:
- systemd units are also deleted (if present)
- the /var/lib/juju/tools folder contents are also removed.

The second change was not included in the back-ported fix but was added to this PR to address a problem (lingering symlinks in the tools folder) that got discovered while doing the bootstrap/cleanup dance several times. This one-line change will also need to be forward-ported to 2.8+

## QA steps

Acquire an instance that you can use as a manual machine. Repeat the following set of
steps 3-4 times to ensure that the machine gets properly cleaned up and provisioned
each time:

```console
$ juju bootstrap manual/ubuntu@$instance_ip --no-gui
$ juju kill-controller -y manual

# SSH into the instance and run the cleanup script
$ sudo /sbin/remove-juju-services
```

## Bug reference
https://bugs.launchpad.net/juju/+bug/1884331

Merge pull request juju#11651 from wallyworld/increment-to-2.8.0

version bump to 2.8.0

Merge pull request juju#11556 from hpidcock/fix-gomod-tests

juju#11556

## Update juju/testing dep and fix test with compare

Updates juju/testing with better HOME detection with tests that abuse env vars.
Fix cloudImageMetadataSuite.TestSupportedArchitectures array compare.

## QA steps

Run unit tests with juju-db snap installed.

## Documentation changes

N/A

## Bug reference

N/A