Adding File.expand_path() when reading SSH keys so '~/.ssh/key.pub' paths work

Adding File.expand_path() when reading SSH keys so '~/.ssh/key.pub' paths may be used to allow users to easily share homestead boxes among other developers working on the same project without having to edit Homestead.yaml

My use case for this was being able to hand other coworkers this same box and allow them to simply vagrant up instead of having to edit files.