When using the upstream cloud images with the "ubuntu" element, they
have universe and multiverse enabled which we don't mirror.
To use the infra mirrors as a DIB_DISTRIBUTION_MIRROR with this
element, we need to be able to skip redirecting to universe and
multiverse, and additionally enable insecure repos (as we don't gpg
sign our mirrors).
Add and document two new variables with the ubuntu element to do this.
This is then setup by the openstack-ci-mirrors element so that we use
local mirrors duing dib functional testing for the "ubuntu" element.
Change-Id: I6ffbde07fa0e103641ee5c5f9d9e854e5b2168dc
c47ee6e121