Fix json-c build failure by adding -Wno-error=unused-value

With the later gcc version from openembedded master json-c fails
to build due to -Werror. This adds a patch that adds -Wno-error=unused
to the cflags.
Add --disable-Werror to telepathy-gabble

This was breaking the build with gcc 4.9 (from poky master).
Use the new gnomeostree-3.14-dizzy branch

This changes the base yocto reference to the gnomeostree-3.14-dizzy
branch which is (temporarily) alexlarsson:poky. I expect this to move
to cgwalters:poky later.

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).