Rewrite check-ceph to no longer use sensu/io, which is absent in 0.13

An attempt to refactor check-ceph.rb to do the same thing that sensu/io previously offered.

I've tried to copy some of the functionality and actual code from sensu/io.rb to ensure it handles things the same way - but it lacks a couple of things that sensu::io offered (process group handling, cross-platform support) as I didn't think these are likely to actually being used in this check.

Happy to make any modifications/changes if needed.