Clean up IPV6 config checks in test_wsgi.

Two tests in test_wsgi require a configured IPV6 interface.
We added a check/skip for this but it wasn't very graceful and
didn't work on OS-X or platforms other than Linux.

This patch implements the checks a bit cleaner using the skip_if
decorator, and combines the linux/path check and configured interface
check.

Change-Id: I950fdfeb8135912faf5dcaabe52fb87abd73dd1e
ensure zeros are written out when clearing volumes

Note O_DIRECT is _not_ used when copying from /dev/zero
and there are issues with enabling that (see 444cd542).
Therefore we arrange to have dd issue an fdatasync()
to ensure the data is persisted, lest it be discarded
from the write cache when the device is unprovisioned.

* cinder/volume/drivers/lvm.py (_copy_volume): Add 'conv=fdatasync'
to the dd option list if O_DIRECT isn't used when clearing
(which it won't as descrived above).

Fixes bug: 1100363
Change-Id: I76789557754ebaeb6d52bb34548a2ef17808fbf6
Improve error message for missing NFS share config

If FLAGS.nfs_share_config is not defined, the error will reference
the name of the config option to set.

If the nfs_share_config path doesn't exist, the error will reference
the path that was configured.

This will hopefully simplify troubleshooting of NFS configuration.

Change-Id: Ica6a60a63330554dc880f36962f3a081926ce5cf
Merge "Clean up IPV6 config checks in test_wsgi."
Merge "ensure zeros are written out when clearing volumes"
Merge "Improve error message for missing NFS share config"