lxd/images: pass publish expiration date to Export(); fallback to metadata.yaml expiration date
lxd/images: pass publish expiration date to Export(); fallback to metadata.yaml expiration date

Signed-off-by: Max Asnaashari <291db2cb5046c5ac435c5cc205a1b9bc80a27f4b@canonical.com>

lxd/images: handle image expiration date when exported from instance

Signed-off-by: Max Asnaashari <291db2cb5046c5ac435c5cc205a1b9bc80a27f4b@canonical.com>

stgraber/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd