.travis.yml: use a known-working version of lxd (#643)

The latest lxd 4.7 snap causes failures; while we're diagnosing those
and addressing the issues (either upstream or in our code), pin at lxd
4.6 so we can continue to land PRs in cloud-init trunk.

.travis.yml

@@ -64,7 +64,7 @@ matrix:
             # pylxd talks only to the lxd from snap
             - sudo apt remove --purge lxd lxd-client
             - sudo rm -Rf /var/lib/lxd
-            - sudo snap install lxd
+            - sudo snap install --channel=4.6/stable lxd
             - sudo lxd init --auto
             - sudo mkdir --mode=1777 -p /var/snap/lxd/common/consoles
             # Move any cached lxd images into lxd's image dir