workaround for tycho bug 409912

Even though bug 409912 [1] is marked as fixed in tycho version 0.18,
it apparently is not. To workaround, explicitly set default groupId
org.apache.maven.plugins for maven-assembly-plugin.

[1] https://bugs.eclipse.org/bugs/show_bug.cgi?id=409912

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>