Update gosdn.clab.yaml

The containerlab topology does now contain two additional centos clients. Additionally the mgmt_ipv4 addresses for each node has 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 accessable for danet group members and everyone else has to download it from the Arista-Homepage.

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.