Fix i18n import issue when using _ in storage.py

Essentially, one of the error messages (which is rarely hit), uses
the underscore (_) from nova.i18n, but it wasn't imported into the
module.

Change-Id: If72f5d16943602f2479c7558324b2ce76cbeed7b

devstack/tempest-dsvm-lxd-rc

@@ -90,4 +90,4 @@ export DEVSTACK_GATE_TEMPEST_REGEX="$r"
 
 # set the concurrency to 1 for devstack-gate
 # See: https://bugs.launchpad.net/nova-lxd/+bug/1790943
-export TEMPEST_CONCURRENCY=1
+#export TEMPEST_CONCURRENCY=1

nova/virt/lxd/storage.py

@@ -17,11 +17,13 @@ import os
 from oslo_config import cfg
 from oslo_utils import fileutils
 from nova import exception
+from nova import i18n
 from nova import utils
 from nova.virt import driver
 
 from nova.virt.lxd import common
 
+_ = i18n._
 CONF = cfg.CONF