pin the base image and the image tag

Recently, the ubuntu 16.04 is out and it crashed kolla when using ubuntu:lastest to
build the images.

even though kolla support multi base-tag, the kolla will failed when using other
base-tag except for centos:7, ubuntu:14.04, rhel:7.
And it is also hard to support all kind of the image tag.

So I support that kolla should restrict the base-tag. the lastest tag is mutable and
we should not use it, especially in the stable branch. When using a mutable image,
it is never a *stable* release.