Use key strings defined in models in callback/lava/boot handler
Use key strings defined in models in callback/lava/boot handler

Use strings defined in models rather than hard-coded strings in the
LAVA boot callback handler.

Signed-off-by: Guillaume Tucker <e7e20b7c30f19d4ca6b81319ef81bf200369d137@collabora.com>

kernelci/kernelci-backend,kernelci/kernelci-backend