provider/lxd: validate results of DefaultPools

[axw]

Description of change

Don't return invalid storage pool config from
DefaultPools, as it'll cause bootstrap to fail.
If the "zfs" tool is unavailable, then validation
will fail. Also, add a "lxd-btrfs" pool while
we're there.

QA steps

1. make sure "btrfs-tools" is not present on the machine
2. juju bootstrap localhost
3. juju storage-pools
   (should not show "lxd-btrfs" as a pool)
4. juju destroy-controller -y localhost
5. sudo apt install btrfs-tools
6. juju bootstrap localhost
7. juju storage-pools
   (should show "lxd-btrfs" as a pool; there should be a corresponding "juju-btrfs" pool in LXD)

Documentation changes

None.

Bug reference

None.

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju