Fixed slaves_unreachable metric.
Fixed slaves_unreachable metric.

While a gauge for slaves_unreachable was created in master metrics,
it was not added to the metrics actor and hence was not tracked by
the metrics subsystem.

Review: https://reviews.apache.org/r/61032/

andschwa/mesos,jpeach/mesos,asamerh4/mesos,dforsyth/mesos,verizonlabs/mesos,andschwa/mesos,reneploetz/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,jpeach/mesos,andschwa/mesos,zmalik/mesos,verizonlabs/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,verizonlabs/mesos,andschwa/mesos,verizonlabs/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,asamerh4/mesos,asamerh4/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,gsantovena/mesos,gsantovena/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,verizonlabs/mesos,jpeach/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,asamerh4/mesos,verizonlabs/mesos,gsantovena/mesos,asamerh4/mesos,gsantovena/mesos,zmalik/mesos,zmalik/mesos,jpeach/mesos,kaysoky/mesos,andschwa/mesos,asamerh4/mesos,abudnik/mesos,zmalik/mesos,gsantovena/mesos,verizonlabs/mesos,zmalik/mesos,Gilbert88/mesos,zmalik/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,asamerh4/mesos,zmalik/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos