Don't check closed elasticsearch indices

Sometimes, we'll have elasticsearch indexes in the cluster which have
been closed (ie, data unloaded from memory, no access allowed except for
reopen or delete).  These will be deleted after a while.  Update the
nagios check to ignore such indices, since otherwise they'll give
spurious errors.
Merge pull request #6 from alphagov/ignore_closed_elasticsearch_indices

Don't check closed elasticsearch indices