Ease configuring extra specs via env vars

Ease configuring extra specs via env vars

Ease configuring flavor extra specs, as well as
image extra specs.

Set environment variables for image_properties and
flavor extra_specs for ovs|fdio scenario from the start.

Image extra_properties and flavor extra_specs
will continue to get updates from functest config file
but so will from IMAGE_PROPERTIES and FLAVOR_EXTRA_SPECS
env variables.

Change-Id: Ie1c69e66d2e2e158915d214402fe44d9991c56d3
Signed-off-by: Delia Popescu <delia.popescu@enea.com>
(cherry picked from commit 1a00b9de56ef65eef2a0b269a514cb25491ac54b)