Refine build environment.

  * Move IT modules under "it" directory.
  * Use maven plugins specified by odlparent.
    * Although odlparent specifies maven-javadoc-plugin 2.10.1,
      we use 2.9.1 because it breaks "mvn site".
  * Skip karaf features test if "vtn.karaf.skip" property is true.
  * dist/karaf: Do nothing on "mvn site".

Change-Id: Ie734ee8be064139541d74f35d40a02a1de911a94
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>