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.

Without this patch, the check is currently failing with:

    Unhandled Python exception: KeyError(u'government-2014-05-01t15:10:20z-4d3f0e37-5c9e-4de2-b77e-5306ac761fc8',)