Use composite builds in functional tests

[mr-archano]

Follow-up work of #276. Aim is to avoid to rely on the java-gradle-plugin magic to inject the classpath of the functional tests with a local copy of the plugin, and use composite builds instead.
The main advantage of this is that the test projects generated as part of the functional tests can be then re-built and debugged without issues.

Implementation details

• A synthetic gradle project (plugin/functional-tests/src/test/gradle) has been introduced to be used along with composite builds
• In order to avoid issues with the publication configuration the buildscripts in :core have been re-arranged:
  • when :core is included in the main Gradle project, its buildscript will include the Bintray configuration, see plugin/core/publish.gradle
  • when :core is included via composite build in the functional tests will instead be using plugin/core/build.gradle as build file
• the templates used to generate the gradle projects for the functional tests now ship with a composite build configuration ensuring that they build against a local copy of the bintray-release plugin without any need of deployments anywhere.