Jenkins: Set number of pytest processes via environment variable (#2925)

tests/jenkins/Jenkinsfile

@@ -16,7 +16,9 @@ pipeline {
     timeout(time: 1, unit: 'HOURS')
   }
   environment {
+    PYTEST_PROCESSES = "${PYTEST_PROCESSES ?: "auto"}"
     PYTEST_ADDOPTS =
+      "-n=${PYTEST_PROCESSES} " +
       "--tb=short " +
       "--color=yes " +
       "--driver=SauceLabs " +