This commit replaces Spring Boot's basic dependency management support
with separate dependency management plugin. This has a number of
benefits including:
1. A Maven bom can be used rather than a custom properties file
2. Dependency management is applied transitively rather than only to
direct dependencies
3. Exclusions are applied as they would be in Maven
4. Gradle-generated poms are automatically configured with the
appropriate dependency management
Closes gh-2133
Spring Buildmaster
Andy Wilkinson
Phillip Webb
Daniel Fullarton