Move OS class to its own files

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add OS.__repr__() and OS.to_dict()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Remove default for --os-type

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Query os_type and os_version in lock_many()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Use os_type and os_version in lock_machines()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>