Divide _cleanup_namespaces for easy extensibility

This division of the function to 2 different functions allows for
easier overwriting in the l3 test agent used by the HA functional
tests, and later by the integration tests.

Related-bug: #1374947
Change-Id: I8f277759747c8a142f5c023aa3511b00a886348c
Merge "Divide _cleanup_namespaces for easy extensibility"