Add log.go back to core of e2e test framework

We tried to separate logger functionality as subpackage of e2e test
framework, but we've recognized that should be core functionality
and we should keep it as core of e2e test framework after facing
circular dependency issues.
So this adds log.go back to core of e2e test framework. In addition,
this makes volume sub package use the core logger as a sample.

test/e2e/framework/log/logger.go

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+// Package log should be removed after switching to use core framework log.
 package log
 
 import (