Add mirror host for images.linuxcontainers.org reverse proxy
Add mirror host for images.linuxcontainers.org reverse proxy

In I8336aeb85689cfaac8a794a5c3f5c62273d8ded0 a reverse proxy
for images.linuxcontainers.org was added to the infra services.

This adds a record in the mirror host env vars so that it's
easy to know which host name to use when executing lxc-create.

eg: lxc-create --template download --name test1 -- --server ${NODEPOOL_LXC_IMAGE_PROXY} --dist ubuntu --release xenial --arch amd64

Change-Id: I457fb98c0aa4e4b4a77acbc77a927a94643b5d65

openstack-infra/project-config,openstack-infra/project-config