Allow overriding ostree-build-yocto

If you create a $workdir/overrides/ostree-build-yocto it will be used
instead of the standard ostree-build-yocto.

This change allow us to use the python27 and git19 scls on rhel6
by having an override file that does something like:

 #!/bin/sh
scl enable python27 git19 "/PREFIX/lib/gnome-continuous/ostree-build-yocto $*"

Where PREFIX is where you installed gnome-continuous.
Use the new gnomeostree-3.14-dizzy branch

This changes the base yocto reference to the gnomeostree-3.14-dizzy
branch in gnome:gnome-continuous-yocto.

Additionally it does a few poky specific changes:
* Drop ca-certificates from manifest in favour of the one
  from poky. This fixes a directory name conflict when replacing
  the poky one with the ostree build one. And anywy, the poky one
  is probably better maintained.
* Change refereces from "tmp-eglibc" to "tmp-glibc" as this got
  directory got renamed in poky (due to switching to glibc).
Bump epoch and rebuild all

This is needed for the new yocto base. Don't want
to keep modules linked against old versions of libs.