Remove ubuntu xenial image building

This change removes the ubuntu xenial image building support since
it's quite old at this point and most services have moved on to
newer releases.

Change-Id: I5b87806d2f07c4d49ac994c0bda8d1e154e02e1d

dockerfiles/ubuntu_xenial/Dockerfile

@@ -1,21 +0,0 @@
-ARG FROM=ubuntu:xenial
-FROM ${FROM}
-
-ARG UBUNTU_URL=http://archive.ubuntu.com/ubuntu/
-ARG CLOUD_ARCHIVE_URL=http://ubuntu-cloud.archive.canonical.com/ubuntu/
-ARG CEPH_URL=http://download.ceph.com/debian-luminous/
-ARG ALLOW_UNAUTHENTICATED=false
-ARG PIP_INDEX_URL=https://pypi.python.org/simple/
-ARG PIP_TRUSTED_HOST=pypi.python.org
-ENV PIP_INDEX_URL=${PIP_INDEX_URL}
-ENV PIP_TRUSTED_HOST=${PIP_TRUSTED_HOST}
-
-COPY sources.list /etc/apt/
-COPY cloud-archive.gpg ceph.gpg /etc/apt/trusted.gpg.d/
-RUN sed -i \
-        -e "s|%%UBUNTU_URL%%|${UBUNTU_URL}|g" \
-        -e "s|%%CLOUD_ARCHIVE_URL%%|${CLOUD_ARCHIVE_URL}|g" \
-        -e "s|%%CEPH_URL%%|${CEPH_URL}|g" \
-        /etc/apt/sources.list
-RUN echo "APT::Get::AllowUnauthenticated \"${ALLOW_UNAUTHENTICATED}\";" \
-         > /etc/apt/apt.conf.d/allow-unathenticated

dockerfiles/ubuntu_xenial/sources.list

@@ -1,6 +0,0 @@
-deb %%UBUNTU_URL%% xenial main universe
-deb %%UBUNTU_URL%% xenial-updates main universe
-deb %%UBUNTU_URL%% xenial-backports main universe
-deb %%UBUNTU_URL%% xenial-security main universe
-deb %%CEPH_URL%% xenial main
-deb %%CLOUD_ARCHIVE_URL%% xenial-updates/pike main