fixes #62

The long-deprecated (since Ansible 2.4) use of include is
no more.

Until now, if the .sql file contained invalid data, psql would
choke on it, but still return an exit code of 0.
This is very misleading.

We need to pass `-v ON_ERROR_STOP=1` to make it exit
with a proper error exit code when failures happen.

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.

This also raises the default from 5 seconds to 15 seconds.

Regression since 3fd6fd647f

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 cbee084ac19b6, 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).