Fix temporary directory cleanup in core/operations

Liberate the tempDir assignment so the cleanup runs with an initialized
value.

Change-Id: I9eeb42475fa7b7dc4f918801ef73ad1016b393c5
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>

core/operations/system_test.go

@@ -44,7 +44,7 @@ var _ = Describe("System", func() {
 
 	BeforeEach(func() {
 		var err error
-		tempDir, err := ioutil.TempDir("", "opssys")
+		tempDir, err = ioutil.TempDir("", "opssys")
 		Expect(err).NotTo(HaveOccurred())
 
 		generateCertificates(tempDir)