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).