try to run mvn verification concurrently

Change-Id: Ie7c4128ba7d383cd896dac1f94d236cb2e9e56d2
diff --git a/jjb/onos/onos-yang-tools-verify.sh b/jjb/onos/onos-yang-tools-verify.sh
index e406f68..bface83 100755
--- a/jjb/onos/onos-yang-tools-verify.sh
+++ b/jjb/onos/onos-yang-tools-verify.sh
@@ -3,4 +3,5 @@
 # exit on errors
 set -eu -o pipefail
 
-mvn -B clean install
+# try concurrent build, then fallback to sequential on failure
+mvn -B clean install -T 1C || mvn -B clean install