refactor megaraid facts + add fact tests

* stop checking for megaraid `/dev/*` files and instead let megacli tell us if it can find any raid controllers
* use blockdevice facts instead of exec'ing `lsscsi`, inspired by Kyle Anderson's [gist](https://gist.github.com/solarkennedy/7606943)
* split each fact into it's own .rb file
* add rspec test coverage of all facts
* remove non-functioning freebsd facts completely