Fix argument handling in oslo_run_cross_tests

If there are no arguments the shift call fails, aborting before we can
report the proper usage instructions. Update the script to check the
argument count before trying to do anything with them.

Change-Id: I3eb7cdf96dcbe6368e2ad0fbc9d0514798292e8e

tools/oslo_run_cross_tests

@@ -9,6 +9,27 @@
 # Fail the build if any command fails
 set -e
 
+function usage {
+    cat - <<EOF
+ERROR: Missing argument(s)
+
+Usage:
+
+  oslo_run_cross_tests PROJECT_DIR VIRTUAL_ENV [POSARGS]
+
+Example, run the python 2.7 tests for python-neutronclient:
+
+  oslo_run_cross_tests /opt/stack/python-neutronclient py27
+  oslo_run_cross_tests /opt/stack/nova py27 xenapi
+
+EOF
+    exit $1
+}
+
+if [[ $# -lt 2 ]]; then
+    usage 1
+fi
+
 project_dir="$1"
 shift
 venv="$1"
@@ -17,20 +38,7 @@ posargs="$*"
 
 if [ -z "$project_dir" -o -z "$venv" ]
 then
-    cat - <<EOF
-ERROR: Missing argument(s)
-
-Usage:
-
-  $0 PROJECT_DIR VIRTUAL_ENV [POSARGS]
-
-Example, run the python 2.7 tests for python-neutronclient:
-
-  $0 /opt/stack/python-neutronclient py27
-  $0 /opt/stack/nova py27 xenapi
-
-EOF
-    exit 1
+    usage 2
 fi
 
 # Set up the virtualenv without running the tests