Make LAVA_CREDENTIALS customizable in jobs

This will allow for different submission profiles, some account may be
able to submit to certain devices, and not others, therefore allowing
disabling of certain device-types if required.

Change-Id: I2f96024d91ae97432693560fb37fb4da4674150b
diff --git a/tf-m-nightly-code-coverage.yaml b/tf-m-nightly-code-coverage.yaml
index 29c2105..65d94a7 100644
--- a/tf-m-nightly-code-coverage.yaml
+++ b/tf-m-nightly-code-coverage.yaml
@@ -91,6 +91,10 @@
       - string:
           name: 'CODE_COVERAGE_EN'
           default: 'TRUE'
+      - string:
+          name: 'LAVA_CREDENTIALS'
+          default: 'LAVA_CREDENTIALS'
+          description: "Name of Jenkins credential for authentication with LAVA."
     pipeline-scm:
       scm:
         - git: