Tweaking a couple of things so they work better

Addressing the following issues:

* `node` executable not found - ironically, Joyent puts `node` in a special place.  Attempting to find it if it's not in `$PATH`, and installing it if it cannot be found
* Which check scripts are used is determined by `node['platform']`.  When Ubuntu, the wrong checks were being used because it was looking for a directory named `ubuntu/` and it could only find `linux/` - fixed by making `ubuntu/` and `centos/` dirs
* Disk check on SmartOS was returning bogus results in some cases.  It was using the wrong version of `df`, so to fix, `/opt/local/gnu/bin` was added to `$PATH`
* also just adding some additional information to the checks