Add a feature to be able to add layers (globally rather than project specific) to Toaster from a pre-defined list of layers in a file which is picked up at start up.

Pre-defined layers would be
 - A layer name
 - A file location that the layer can be found in
 - What release that layer is valid for

This requires that toaster can:
 - Use layers from directories which are both git and non git sources (such as an extracted tar archive)
 - Load a config file to populate the layer information


History:

Originally we tried using the toasterconf.json but it is quite tricky because it's really an "internal" file to toaster for the purposes of switching between poky and openembedded setups. We also have the issue that toaster assumes that the layers are in git repositories. Work to avoid that assumption by Sujith has been done here and can be referred to when we implement this feature:

http://git.yoctoproject.org/cgit/cgit.cgi/poky-contrib/log/?h=sujith/Import-external-layers-toaster-and-error-fix

Proposed solution:

- Add a management command to load layers e.g. manage.py loadlayers yourlayers.conf
- Fix toaster git repository assumptions