[v23.2.x] cluster/config_manager: protect cfg.get() behind cfg.contains()

[vbotbuildovich]

Backport #15840 to branch v23.2.x. Note that this issue was created as a placeholder, since the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v23.2.x
git checkout -b vbotbuildovich/backport-15840-v23.2.x-705
git cherry-pick -x d2dcba9745a2c6e20fdaf2ff260bd57773027504 d02b426794a372c551845ee9532db21d77332f84 f891fd6feed481253a07636496d96883496ba76f

git push origin vbotbuildovich/backport-15840-v23.2.x-705
gh pr create \
  --title "[v23.2.x] cluster/config_manager: protect cfg.get() behind cfg.contains()" \
  --base "v23.2.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-15840-v23.2.x-705" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "Lazin,dotnwat" \
  --milestone "v23.2.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/15840 "