Attach a label to servers with their desired deletion date

[bastelfreak]

This then enables us to periodically scan for images with a unix timestamp in the label that's smaller than now() and delete them.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (41229a0) 92.30% compared to head (a745311) 93.10%.

:exclamation: Current head a745311 differs from pull request most recent head 97e65a8. Consider uploading reports for the commit 97e65a8 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #29 +/- ## ========================================== + Coverage 92.30% 93.10% +0.79% ========================================== Files 3 3 Lines 78 87 +9 ========================================== + Hits 72 81 +9 Misses 6 6 ``` | [Files](https://app.codecov.io/gh/voxpupuli/beaker-hcloud/pull/29?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli) | Coverage Δ | | |---|---|---| | [lib/beaker/hypervisor/hcloud.rb](https://app.codecov.io/gh/voxpupuli/beaker-hcloud/pull/29?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=voxpupuli#diff-bGliL2JlYWtlci9oeXBlcnZpc29yL2hjbG91ZC5yYg==) | `95.23% <100.00%> (+0.79%)` | :arrow_up: |

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[bastelfreak]

This requires https://github.com/tonobo/hcloud-ruby/pull/84

[ekohl]

I do see failing tests. Perhaps split up the PR?

[bastelfreak]

@ekohl I pulled the second change into https://github.com/voxpupuli/beaker-hcloud/pull/30