Many fixtures have similar code for writing the pid & ports files or
for handling HTTP requests. This commit adds an AbstractHttpFixture
class in the test framework that can be extended for specific testing purposes.
This pull request replaces the jvm-example plugin (from the jvm/site plugins era) by two new plugins: a custom-settings that shows how to register and use custom settings (including secured settings) in a plugin, and rest-handler plugin that shows how to register a rest handler.
The two plugins now reside in the plugins/examples project. They can serve as sample plugins for users, a special attention has been put on documentation. The packaging tests have been adapted to use the custom-settings plugin.