map-order sensitive test in md/juju/storage needs to be fixed

This is consistently failing on vivid and probably on other platforms. See http://data.vapour.ws/juju-ci/products/version-2483/run-unit-tests-vivid-amd64/build-297/consoleText.

----------------------------------------------------------------------
FAIL: poollist_test.go:114: PoolListSuite.TestPoolListTabularSorted

poollist_test.go:125:
    s.assertValidList(
        c,
        []string{"--name", "myaw", "--name", "xyz", "--name", "abc",
            "--format", "tabular"},
        `
    NAME PROVIDER ATTRS
    abc testType one=true two=well three=maybe
    myaw testType one=true two=well three=maybe
    xyz testType one=true two=well three=maybe

    `[1:])
poollist_test.go:133:
    c.Assert(obtained, gc.Equals, expected)
... obtained string = "" +
... "NAME PROVIDER ATTRS\n" +
... "abc testType three=maybe one=true two=well\n" +
... "myaw testType one=true two=well three=maybe\n" +
... "xyz testType one=true two=well three=maybe\n" +
... "\n"
... expected string = "" +
... "NAME PROVIDER ATTRS\n" +
... "abc testType one=true two=well three=maybe\n" +
... "myaw testType one=true two=well three=maybe\n" +
... "xyz testType one=true two=well three=maybe\n" +
... "\n"

OOPS: 23 passed, 1 FAILED
--- FAIL: TestAll (0.07 seconds)
FAIL
FAIL github.com/juju/juju/cmd/juju/storage 0.388s