detect failure of 'keystone token-get'

When 'keystone token-get' fails, the caller can't detect the failure.
This cause troulbe shooting a bit complicated.

Change-Id: I3c58c5fd0e92a87e87546ea797904e08646a1097
Closes-Bug: #1238412
diff --git a/exercises/neutron-adv-test.sh b/exercises/neutron-adv-test.sh
index abb29cf..e0c37ef 100755
--- a/exercises/neutron-adv-test.sh
+++ b/exercises/neutron-adv-test.sh
@@ -102,6 +102,7 @@
 # and save it.
 
 TOKEN=`keystone token-get | grep ' id ' | awk '{print $4}'`
+die_if_not_set $LINENO TOKEN "Keystone fail to get token"
 
 # Various functions
 # -----------------