We've had that logic in 2 places so far, leading to duplication
and a maintenance burden.
In the future, we'll also have an import-postgres feature,
which will also need Postgres version detection,
leading to more benefit from that logic being reusable.
Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).
Since cbee084ac1, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.
This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).
