cephadm-adopt: Fixes binding network for alertmanager (backport #7576)

[mergify]

Alertmanager was bind to default * network instead of grafana_server_addr as it was before. Now on if grafana_server_addr is defined, it will be bind to that network.

---

This is an automatic backport of pull request #7576 done by Mergify.

[mergify]

Cherry-pick of 0bf3398 has failed:

On branch mergify/bp/stable-7.0/pr-7576
Your branch is up to date with 'origin/stable-7.0'.

You are currently cherry-picking commit 0bf339877.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   infrastructure-playbooks/cephadm-adopt.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[asm0deuz]

LGTM