Ensure DataSource can load database driver

Update DataSource conditional to ensure that the driver class
can actually be loaded by the DataSource. This fixes an issue when
deploying a classic WAR where `org.apache.tomcat.jdbc.pool.DataSource`
is found the parent classloader but the database driver cannot be loaded
because is included as a local `/lib` dependency.
Support single config from ServletInitializer

Update SpringBootServletInitializer with separate getConfigClass() and
getAdditionalConfigClasses() methods. This change makes it easier to
use the SpringBootServletInitializer with the common use case of a
single config class.