Use ubuntu xenial box.
Use ubuntu xenial box.

Now that Ubuntu has kernel related features
needed for Docker, use it.

Signed-off-by: Gurucharan Shetty <a1872e333d0e52644f6125da2276530f7ebe5e77@ovn.org>

shettyg/ovn-docker,shettyg/ovn-docker,shettyg/ovn-docker