TRITON-2415 Add lxd to list of valid image types

[travispaul]

Prevent tests from failing if lxd images (for LinuxCN) are installed.

After making this change, multiple tests that were failing with the following error no longer fail:

not ok 174 expected image.type: lxd
  ---
    operator: ok
    expected: true
    actual:   false
    at: checkImage (/opt/smartdc/cloudapi/test/images.80.test.js:48:7)
    stack: |-
      Error: expected image.type: lxd
          at Test.assert [as _assert] (/opt/smartdc/cloudapi/node_modules/tape/lib/test.js:228:54)
          at Test.bound [as _assert] (/opt/smartdc/cloudapi/node_modules/tape/lib/test.js:80:32)
          at Test.assert (/opt/smartdc/cloudapi/node_modules/tape/lib/test.js:347:10)
          at Test.bound [as ok] (/opt/smartdc/cloudapi/node_modules/tape/lib/test.js:80:32)
          at checkImage (/opt/smartdc/cloudapi/test/images.80.test.js:48:7)
          at /opt/smartdc/cloudapi/test/images.80.test.js:99:13
          at Array.forEach (native)
          at /opt/smartdc/cloudapi/test/images.80.test.js:98:14
          at parseResponse (/opt/smartdc/cloudapi/node_modules/restify/lib/clients/json_client.js:91:9)
          at IncomingMessage.done (/opt/smartdc/cloudapi/node_modules/restify/lib/clients/string_client.js:167:13)
  ...

I also confirmed that they still pass if lxd images are not present.

I am not yet getting the entirety of the cloudapi tests to pass locally but will tackle those issues in future PRs.