Move to using xenial64 as the base operating system.
Move to using xenial64 as the base operating system.

Change-Id: Ib7071fb0fe1206e7868d89c5418f5655ec77f096

JohnVillalovos/devstack-gate-test,JohnVillalovos/devstack-gate-test,JohnVillalovos/devstack-gate-test