devstack-gate is still used for grenade jobs and restricts
ara<1.0.0 and defaults ANSIBLE_VERSION=2.5.7.
ARA 0.16.6 dropped support for ansible < 2.6 . As such,
grenade jobs are failing because ara 0.16.6 cannot be installed.
This bumps the ANSIBLE_VERSION default to 2.7.14 which is current
as of Ubuntu Xenial 16.04 which should be sufficient for the
branches still supported in upstream OpenStack CI (mainly Ocata+).