What
---
- Refactor the common patroni dependencies.
- Add README file showing off how to use the library.

Why
---
`patroni` is a downstream dependency for a lot of services such as
`api`, `git`, `web`. This resulted into a lot of duplication of code.

Testing
---
To make sure there are no diffs between the `master` branch and this
branch run the following:

```sh
git switch master
./alertmanager/generate.sh && cp alertmanager/alertmanager.yml /tmp/alertmanager.old
git switch refactor/extract-depend-on-patroni
./alertmanager/generate.sh && cp alertmanager/alertmanager.yml /tmp/alertmanager.new
diff /tmp/alertmanager.old /tmp/alertmanager.new
```

Reference: gitlab-com/gl-infra/reliability#15998


Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>