standardize database_configuration to a hash

make connection_url_to_hash a class method

This als prevents loading database.yml if it doesn't exist
but DATABASE_URL does
test DATABASE_URL without database.yml around
run the load_structure test in the test environment
update changelog
Merge pull request #9120 from hone/db_migrate_with_database_url

fix rake db:migrate when used with ENV['DATABASE_URL']