Switch linter jobs to ubuntu 18.04 minion by default

Change-Id: Iff7d58d164462770039f47411891b861fcf8029f
Issue-ID: INT-1478
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>
diff --git a/jjb/testsuite/testsuite.yaml b/jjb/testsuite/testsuite.yaml
index f93a1fa..7d0eb5d 100644
--- a/jjb/testsuite/testsuite.yaml
+++ b/jjb/testsuite/testsuite.yaml
@@ -63,8 +63,7 @@
     project-name: 'testsuite'
     python-version: python3
     jobs:
-      - '{project-name}-{stream}-{subproject}-lint':
-          build-node: ubuntu1804-builder-4c-4g
+      - '{project-name}-{stream}-{subproject}-lint'
     subproject:
       - 'yaml':
           tox-dir: '.'