spread (via snap) exec: "lxc": executable file not found in $PATH

_Cannot allocate lxd:ubuntu-16.04: cannot launch lxd container: exec: "lxc": executable file not found in $PATH_

I was looking to start using spread very soon while we're redeploy some ci infra. I've got a fresh Xenial instance. I can launch and list lxc a-ok as my user, but can't get spread to do the examples.

It has the same error whether installed in strict or in dev mode.

```
ubuntu@osci-slavehost:~/temp⟫ tree
.
├── examples
│ └── hello
│ └── task.yaml
└── spread.yaml

2 directories, 3 files

ubuntu@osci-slavehost:~/temp⟫ head -n999 examples/hello/task.yaml spread-hello.yaml spread.yaml
==> examples/hello/task.yaml <==
summary: Greet the planet
environment:
GREETING: Hello
GREETING/bar: Goodbye
SUBJECT/baz: world
execute: |
echo "$GREETING $SUBJECT!"
exit 1

==> spread.yaml <==
project: hello-world

backends:
lxd:
systems:
- ubuntu-16.04

suites:
examples/:
summary: Simple examples
environment:
SUBJECT/foo: sanity
SUBJECT/bar: lunacy

path: /remote/path

ubuntu@osci-slavehost:~/temp⟫ lxc list
+-----------------------+---------+------------------+------+------------+-----------+
| NAME | STATE | IPV4 | IPV6 | TYPE | SNAPSHOTS |
+-----------------------+---------+------------------+------+------------+-----------+
| intergalactic-shellie | RUNNING | 10.0.8.45 (eth0) | | PERSISTENT | 0 |
+-----------------------+---------+------------------+------+------------+-----------+

# ^ lxc unit created by ubuntu user manually

ubuntu@osci-slavehost:~/temp⟫ spread -list
lxd:ubuntu-16.04:examples/hello:baz
lxd:ubuntu-16.04:examples/hello:bar
lxd:ubuntu-16.04:examples/hello:foo
ubuntu@osci-slavehost:~/temp⟫ spread -v
2016/08/26 00:14:57 Found /home/ubuntu/temp/spread.yaml.
2016/08/26 00:14:57 Allocating lxd:ubuntu-16.04...
2016/08/26 00:14:57 Cannot allocate lxd:ubuntu-16.04: cannot launch lxd container: exec: "lxc": executable file not found in $PATH
2016/08/26 00:14:57 Successful tasks: 0
2016/08/26 00:14:57 Aborted tasks: 3
error: unsuccessful run

1 ubuntu@osci-slavehost:~/temp⟫ snap list
Name Version Rev Developer Notes
spread 2016.08.23 11 niemeyer -
ubuntu-core 16.04.1 216 canonical -
```