Add new variables for customising the deployment image in Bifrost

Add new variables, ``kolla_bifrost_dib_elements_extra`` and
``kolla_bifrost_dib_env_vars_extra``, in
``${KAYOBE_CONFIG_PATH}/bifrost.yml`` for setting additional
``diskimage-builder`` elements and environment variables for the disk
image generated by Bifrost. Default elements and environment variables
are now respectively defined by ``kolla_bifrost_dib_elements_default``
and ``kolla_bifrost_dib_env_vars_default``.

Change-Id: I22ab15c42d8db772f1a64f325f9371860491ccfb