Add support for postgres, mariadb/mysql databases (#23)

* Add environment variables for Database configuration.
* Install postgres and mysql dependencies.
* Test refactoring
* Run tests for prosody with postgres and also for prosody with sqlite.
* Add tests that check if postgres or sqlite is used.

tests/tests-prosody_postgres.bats

@@ -0,0 +1,10 @@
+# For tests with pipes see: https://github.com/sstephenson/bats/issues/10
+
+load 'bats/bats-support/load'
+load 'bats/bats-assert/load'
+
+@test "Should use postgres" {
+  run bash -c "sudo docker-compose logs $batsContainerName | grep -E \"Connecting to \[PostgreSQL\] prosody\.\.\.\""
+  assert_success
+  assert_output
+}