Update keepalived dependency (libnl-dev -> libnl3-dev)

As a preparation for the next keepalived release, based on this discussion: https://github.com/acassen/keepalived/pull/1576#issuecomment-636690506 The source code from the current keepalived master branch does not compile anymore with libnl-dev, now libln3-dev is required (it is already preferred according to the sample Dockerfile from the project repo).