toastergui: layer name correlation

toastergui: layer name correlation

This patch modifies how layers are identified and matched.

Layers were primarely organized by the source of layer information,
and Releases were separated by both layer git branches and originating
source of layer information. This setup prevented mixing layers from
different sources for a certain release, which didn't match the way
people use Yocto Project / bitbake.

This patch brings name-based indentification, where layers with the
same name are assumed to be equivalent, in the sense of being able
to substitute one another. To facilitate this identification to
humans, layers are differentiated by GIT URI instead of layer sources,
which was a rather arbitrary abstraction.

Additional changes include modification to models in order accomodate
for the new data structure, and to config file loading to match
the new toasterconf.json layout.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>