Do not secure-delete snapshots for LVM driver

Secure-deleting snapshots cause a kernel hang on an Ubuntu Precise
machine. Although we were already skipping secure-deleting with
an LVM snapshot, this commit makes this more visible by comments
in the code as well as by generating a specific warning in logs.

Fixes bug 1134768 (well, not actually)

Change-Id: I66586bd8144f265ad0471e99f03449d7b130bc86