(FACT-727) Add meaningful operatingsystem fact values for Arista EOS

This commit adds support for Arista EOS within the operating system
set of facts. This includes the `os`, `operatingsystem`, `osfamily`
`operatingsystemmajrelease`, and `operatingsystemrelease` facts.
Merge pull request #806 from whopper/fact-727/arista_linux

(FACT-727) Add meaningful operatingsystem fact values for Arista EOS