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/n1sdp-linux.yaml b/script/lava-templates/n1sdp-linux.yaml
index 0903064..dab89c5 100644
--- a/script/lava-templates/n1sdp-linux.yaml
+++ b/script/lava-templates/n1sdp-linux.yaml
@@ -4,7 +4,7 @@
   job:
     minutes: 30
 
-priority: medium
+priority: ${LAVA_PRIORITY:-medium}
 visibility: public
 context:
   extra_nfsroot_args: ',vers=3'