provider/lxd: test suite panics if lxd not installed

PANIC: provider_test.go:39: providerSuite.TestOpen

[LOG] 0:00.021 DEBUG juju.service discovered init system "upstart" from series "trusty"
[LOG] 0:00.037 DEBUG juju.container.lxd.lxdclient initializing config dir "/tmp/check-2775422040480279449/1/juju-testenv"
[LOG] 0:00.038 DEBUG juju.container.lxd.lxdclient writing config file "/tmp/check-2775422040480279449/1/juju-testenv/config.yml"
[LOG] 0:00.039 DEBUG juju.container.lxd.lxdclient not writing invalid/empty certificate
[LOG] 0:00.039 DEBUG juju.container.lxd.lxdclient loading LXD client config from "/tmp/check-2775422040480279449/1/juju-testenv"
[LOG] 0:00.040 DEBUG juju.container.lxd.lxdclient using LXD remote "local"
provider_test.go:41:
    c.Check(err, jc.ErrorIsNil)
... value *errors.Err = &errors.Err{message:"", cause:(*url.Error)(0xc820466420), previous:(*errors.Err)(0xc82050d2c0), file:"github.com/juju/juju/provider/lxd/provider.go", line:26} ("can't connect to the local LXD server: Get http://unix
.socket/1.0: dial unix /var/lib/lxd/unix.socket: connect: no such file or directory")
... error stack:
        Get http://unix.socket/1.0: dial unix /var/lib/lxd/unix.socket: connect: no such file or directory
        github.com/juju/juju/provider/lxd/lxdclient/client.go:68: can't connect to the local LXD server
        github.com/juju/juju/provider/lxd/lxdclient/client.go:37:
        github.com/juju/juju/provider/lxd/environ_raw.go:67:
        github.com/juju/juju/provider/lxd/environ_raw.go:36:
        github.com/juju/juju/provider/lxd/environ.go:49:
        github.com/juju/juju/provider/lxd/provider.go:26:

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0x49038E)

/home/dfc/go/src/runtime/asm_amd64.s:471
  in call32
/home/dfc/go/src/runtime/panic.go:426
  in gopanic
/home/dfc/go/src/runtime/panic.go:45
  in panicmem
/home/dfc/go/src/runtime/sigpanic_unix.go:24
  in sigpanic
environ.go:137
  in environ.Config
provider_test.go:43
  in providerSuite.TestOpen
/home/dfc/go/src/runtime/asm_amd64.s:471
  in call32
/home/dfc/go/src/reflect/value.go:303
  in Value.Call
/home/dfc/go/src/runtime/asm_amd64.s:1963
  in goexit

----------------------------------------------------------------------
FAIL: provider_test.go:47: providerSuite.TestPrepareForBootstrap