Add nova-lxd storage-pool option for ZFS storage backend

Choosing zfs storage backend for nova-lxd requires specifying
a storage pool name. This option is not in the template.
This adds an option to specify the storage pool name if a variable
has been populated with the name.

Change-Id: Ifb29545c715c93362283399d198ec9b0e77aa630
Closes-Bug: #1655064
(cherry picked from commit f7670e3e43)

templates/lxd-init.sh.j2

@@ -9,4 +9,7 @@
   --network-address={{ lxd_bind_address }} \
   --network-port={{ lxd_bind_port }} \
   --storage-backend={{ lxd_storage_backend }} \
+{% if lxd_storage_pool is defined %}
+  --storage-pool={{ lxd_storage_pool }} \
+{% endif %}
   --trust-password={{ lxd_trust_password }}