spec_helper: set DEFAULT_DATABASE_PORT to 1521

Having undefined DEFAULT_DATABASE_PORT env variable resulted in port 0
instead of 1521. This change is resolving that.
Merge pull request #56 from javornikolov/set-tests-default-db-port-to-1521

spec_helper: set DEFAULT_DATABASE_PORT to 1521
spec_helper: allow customizing db service name

This allows running tests over fully qualified db service
name which doesn't match the database name
schema_spec: connect to service via service name

Use DATABASE_SERVICE_NAME instead of DATABASE_NAME when connecting to
database service directly (i.e. not via TNS).