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).
Merge pull request #55 from javornikolov/enable-running-tests-on-fully-qualified-db-service-name

Allow running tests over fully qualified db service name which doesn't match the database name