Add a few topologies

topologies/README.md

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+# Topologies
+
+This directory contains various topologies useful to be kept.

topologies/abc.yaml

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+---
+links:
+  - domains: ['a', 'b']
+  - domains: ['b', 'c']

topologies/abc_leafv4.yaml

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+---
+links:
+  - domains: ['a', 'b']
+  - domains: ['b', 'c']
+domains:
+  b:
+    enable_v4: false