spec_helper: allow customizing db service name
spec_helper: allow customizing db service name

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

jgebal/ruby-plsql,JDominator/ruby-plsql,jgebal/ruby-plsql,yahonda/ruby-plsql,jgebal/ruby-plsql,yahonda/ruby-plsql,rsim/ruby-plsql,JDominator/ruby-plsql,rsim/ruby-plsql