Simplify Keycloak's configuration of Postgres SSL-related connection settings

[kgudel]

Currently ssl is implemented by tacking ?ssl=true&sslfactory=org.postgresql.ssl.NonValidatingFactory onto the back of the env.POSTGRES_DATABASE.

This is only possible because that environment variable is only used to construct the JDBC url and because it happens to be the last value.

We should have a better way of handling ssl. Such as by including an env.SSL variable that would turn ssl on or off.