Restore logback base.xml behavior

Update the base.xml logback configuration file so that it behaves in
the same way as Spring Boot 1.1.

Fixes gh-1845
Default spring.datasource.jmx-enabled to false

Change the default value of spring.datasource.jmx-enabled to false
to prevent InstanceAlreadyExistsException problems when using the
Spring Test Framework.

Fixes gh-1590