travis: use LXD from 3.0 track

An update to LXD was recently released that changes some of the
networking stuff done by `init` such that Travis broke on us. Even
though this breakage also effects 3.0, we should be using LXD 3.0 in CI
so we don't get other random breakage. Also, stop setting up testbr0,
which broke and doesn't seem to be necessary anymore.

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>

tools/travis/setup_lxd.sh

@@ -35,7 +35,7 @@ apt-get remove --yes lxd lxd-client
 # - Setup snap "core" (3604) security profiles (cannot reload udev rules: exit status 2)
 # but the installation succeeds, so we just ingore it.
 snap install core || echo 'ignored error'
-snap install lxd
+snap install lxd --channel=3.0/stable
 # Wait while LXD first generates its keys. In a low entropy environment this
 # can take a while.
 
@@ -46,5 +46,3 @@ for i in $(seq 12); do
 done
 
 /snap/bin/lxd init --auto
-/snap/bin/lxc network create testbr0
-/snap/bin/lxc network attach-profile testbr0 default eth0