Unmounting ephemeral if it has already been mounted

Creating an instance from a backup fails when using
ephemeral storage (trove_volume_support = False).
When using ephemeral, nova automatically mounts the ephemeral
drive /dev/vdb into /mnt. This causes issues during the restore
from backup when the restore method tries to delete all files from
the mounted device. It gets an error complaining about the
"Device or resource busy".

To solve the problem, we should unmount the ephemeral storage
if it's been already mounted.

Change-Id: I007c1e11274f1f497d26bd9b1be1fc5b0830f84b
Closes-Bug: #1294369
Merge "Unmounting ephemeral if it has already been mounted"