handler: simplify parsing valid location constraint. (#4040)

Separate out validating v/s parsing logic in
isValidLocationConstraint() into parseLocationConstraint()
and isValidLocation()

Additionally also set `X-Amz-Bucket-Region` as part of the
common headers for the clients to fallback on in-case of any
region related errors.

cmd/admin-handlers_test.go

@@ -1346,6 +1346,11 @@ func TestToAdminAPIErr(t *testing.T) {
 }
 
 func TestWriteSetConfigResponse(t *testing.T) {
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(rootPath)
 	testCases := []struct {
 		status bool
 		errs   []error