Fix: softlayer getAssignableLocations missing NL

Was originally part of https://github.com/jclouds/jclouds/pull/147 - see @jdtoy 's comment:

    "NL-NH" is probably most accurate for Amsterdam.

And my response there:

    @jdtoy thanks, but I'll leave it as "NL" for this pull request if that's ok.

    I think in another part of the softlayer integration code we're also extracting the datacenter's iso3166 automatically from the json returned by softlayer, which gives us "country":"NL". There's a test that asserts the extracted string is contained within the set of iso3166 codes hard-coded in the softlayer metadata (which you spotted). So changing the hard-coded value would break the test, until we did some more work...