gradle: Pass bnd gradle plugin classpath to build.gradle

Plugin coordinates are in gradle.properties and ivy repository is setup
in settings.gradle. We do that to use the plugin from the cloudbees
build.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #671 from bjhargrave/gradle-repo-plugin

gradle: Pass bnd gradle plugin classpath to build.gradle