Macros for path calculation to the testing image for deploy.
It accepts only one string parameter -- file extension to be added
to the base image URL.

In case if global variable 'image_base_url' is set -- it is returned
with requested extension instead of on-the-fly calculation.

As a side effect -- it is possible to define the image for the test
via CLI without changing other variables, for instance:
  lqa -g lava/profiles.yaml -t image_base_url:'https://URL/name.img

'

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>