Clean up QTree when deleting volume on NetApp storage box.

Previously, deleting a Cinder volume would leave a QTree on NetApp
storage box. This commit fixes the issue.

Fixes bug 1099414

Change-Id: I95f1d6b802f4abfad421d34f1c000b75fb72c41e
Update osapi_volume_extension default.

Updates the Cinder config default for osapi_volume_extension
so that it only uses the new cinder.api.contrib.standard_extensions
loader.

Previously both the new and old extension loader path were listed
which can cause extensions to be loaded twice thus causing
log WARNINGs to appear in the Cinder API log file.

Also, regenerates the cinder.conf.sample to reflect this change.

Fixes LP Bug #1107963.

Change-Id: I4e060fc8dcc49d8285e0e7db57489d10aa6d193a
Merge "Update osapi_volume_extension default."
Merge "Clean up QTree when deleting volume on NetApp storage box."