Change bootstrapping series back to xenial.

This is a temporary fix. Jenkins slave can't bootstrap a bionic series right now.

Change-Id: I28da224c093a102a964f15bcbd7fadfca771b9af
Signed-off-by: David Garcia <david.garcia@canonical.com>
(cherry picked from commit 7ca1ec63172ee3e98ce9ad56bc925b90ef771b49)
1 file changed