Restructured javadocs build to make groupings more manageable in the light of mavan-javadoc-plugin's silly treatment of package lists.

Change-Id: I771db3399bf8f0463927b2720ff43f862d7f2d68
diff --git a/docs/external-incubator-apis b/docs/external-incubator-apis
new file mode 100644
index 0000000..1e3bd7a
--- /dev/null
+++ b/docs/external-incubator-apis
@@ -0,0 +1,2 @@
+org.onosproject.exp
+org.onosproject.incubator.*