Retry on closing of luks encrypted volume in case device is busy

We're seeing races in the gate on detach of an encrypted volume where
the device is busy so the detach fails, which causes the delete of the
volume on the cinder side to fail because the volume is still in use and
then Tempest times out waiting for the volume to be deleted.

This simply adds a retry to close the encryption device.  I use
attempts=3 since that's what we use for lvremove.

Closes-Bug: #1374458

Change-Id: Ia0667da7dee247ba8c3338b296244b2a71d1045d
(cherry picked from commit eef97cdf4bb7f426d7feb394ef54510db8b1656b)