Update gosdn.clab.yml with additional nodes and link to private container repo

The containerlab topology does now contain two additional centos
clients. Additionally, the mgmt_ipv4 addresses for each node have been
set.

The topoplogy now does look like:

ceos0 <- - - -> ceos1a
  |               |
  |               |
centos1         centos2

The README file has been updated to address the new situation with the
Arista cEOS image location. We've moved the image into a private
repository so that it is only accessible for danet group members and
everyone else has to download it from the Arista Homepage.