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
(cherry picked from commit 163c210388bc17c6e5a891588eb916935e087342)
diff --git a/script/lava-templates/qemu-linux.yaml b/script/lava-templates/qemu-linux.yaml
index 814b20e..57b287b 100644
--- a/script/lava-templates/qemu-linux.yaml
+++ b/script/lava-templates/qemu-linux.yaml
@@ -1,6 +1,6 @@
 device_type: qemu
 job_name: qemu-linux-${test_config}
-priority: medium
+priority: ${LAVA_PRIORITY:-medium}
 visibility: public
 
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