refactor megaraid facts + add fact tests

[jhoblitt]

• 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
• split each fact into it's own .rb file
• add rspec test coverage of all facts
• remove non-functioning freebsd facts completely