lava-templates: Allow to override job priority

The idea is to pass it as a param from a top-level job, e.g. make
tf-a-daily (background job) to use "low" priority, to give more
priority to "interactive" jobs, like patch testing.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I216d5f9693512945c95bb55c14090d575d82696c
8 files changed