This is mostly so as to avoid referring to variables from other roles,
like `matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container`.

This should come (and already does) from group_vars/matrix_servers

`matrix_media_repo_identifier` is already defined in the role defaults,
which is a better role to have it anyway.

* Updated MMR docs with updated fields in main.yaml

* Removed uneeded placeholder db password

This is backward-compatible with what we had before. We're not changing
the SSL mode - just making it configurable.

Most components are defaulting to `sslmode=disable`, while some
(`matrix-bot-matrix-reminder-bot` and others) do not specify an `sslmode` at all.

We're making sslmode configurable, because certain external Postgres
servers may be configured to require SSL encryption.
In such cases `sslmode=disable` does not work and needs to be changed to
`sslmode=require` or something else (`verify-ca`, `verify-full`, etc).