Stabilize openstack_test when running against real cloud

in TestReadConfig, we are setting some env vars for testing if
we read them back properly. However this interferes with running
the unit test harness against a real openstack cloud where we
source the OS_* environment variables. Adding code here to save
and reset variables.

pkg/cloudprovider/providers/openstack/openstack_test.go

@@ -88,6 +88,11 @@ func TestReadConfig(t *testing.T) {
 		t.Errorf("Should fail when no config is provided: %s", err)
 	}
 
+	// Since we are setting env vars, we need to reset old
+	// values for other tests to succeed.
+	env := clearEnviron(t)
+	defer resetEnviron(t, env)
+
 	os.Setenv("OS_PASSWORD", "mypass")
 	defer os.Unsetenv("OS_PASSWORD")
 
@@ -681,3 +686,24 @@ func TestToAuth3Options(t *testing.T) {
 		t.Errorf("DomainName %s != %s", ao.DomainName, cfg.Global.DomainName)
 	}
 }
+
+func clearEnviron(t *testing.T) []string {
+	env := os.Environ()
+	for _, pair := range env {
+		if strings.HasPrefix(pair, "OS_") {
+			i := strings.Index(pair, "=") + 1
+			os.Unsetenv(pair[:i-1])
+		}
+	}
+	return env
+}
+func resetEnviron(t *testing.T, items []string) {
+	for _, pair := range items {
+		if strings.HasPrefix(pair, "OS_") {
+			i := strings.Index(pair, "=") + 1
+			if err := os.Setenv(pair[:i-1], pair[i:]); err != nil {
+				t.Errorf("Setenv(%q, %q) failed during reset: %v", pair[:i-1], pair[i:], err)
+			}
+		}
+	}
+}