Bug 1087456 part1: Update base image to CentOS 7.

releng uses CentOS for CI. In order to reuse their packages, we also
move from Ubuntu to CentOS for our base images.
Bug 1087456 part2: change builder image from "gecko-base" to "base".

builder image is now based on the "base" image that lives on gecko
tree.
Bug 1087456 part3: create repositories parent dirs if they don't exist.

Older versions of mercurial cannot create inner directories if the
parents don't exist.
Bug 1087456 part4: use build-setup vars to reference repos.