bots: Fix image-customize for `$TEST_DATA` and absent images
bots: Fix image-customize for `$TEST_DATA` and absent images

In this case it currently fails like this:

```
$ bots/image-customize -v -i bash fedora-27
qemu-img: [..]/test/images/fedora-27.qcow2: Could not open '[..]/bots/images/fedora-27-e49f[..].qcow2':
No such file or directory
```

Fix this by calling `image-download` first, which will either download
missing images or create the symlink to the actual image in
`$TEST_DATA`.

 Closes #8528
cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,SotolitoLabs/cockpit,martinpitt/cockpit,andreasn/cockpit,stefwalter/cockpit,mareklibra/cockpit,mvollmer/cockpit,stefwalter/cockpit,garrett/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mareklibra/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,SotolitoLabs/cockpit,garrett/cockpit,cockpituous/cockpit,mareklibra/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,deryni/cockpit,stefwalter/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit,andreasn/cockpit,martinpitt/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,mareklibra/cockpit,andreasn/cockpit,cockpituous/cockpit,stefwalter/cockpit,martinpitt/cockpit,SotolitoLabs/cockpit,cockpituous/cockpit,andreasn/cockpit,garrett/cockpit,mareklibra/cockpit,mvollmer/cockpit,cockpit-project/cockpit,andreasn/cockpit,SotolitoLabs/cockpit,cockpit-project/cockpit,stefwalter/cockpit,mareklibra/cockpit,deryni/cockpit,SotolitoLabs/cockpit,deryni/cockpit,stefwalter/cockpit,stefwalter/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,SotolitoLabs/cockpit,mareklibra/cockpit,mvollmer/cockpit,SotolitoLabs/cockpit