toaster: new layer checkout logic

toaster: new layer checkout logic

This patch implements a new layer checkout logic that brings more
flexibility in getting layers under different commits work with
Toaster.

The new hibrid logic will checkout separately each layer and commit id;
the task execution will be delegated to the checkedout bitbake, but the
data logger will be executed from the current toaster version as to
bring in enough data to sustain the updates in UI models.

[YOCTO #7171]
[YOCTO #6892]

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