Rename Node.distro -> Node.os_type

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

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add alembic migration for Node table changes

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