Refactor core network handling in CI, harmonize code

Prior to change set, there is a highly heterogeneous situation for core network management in CI code: some log collection is done from groovy files (when it could be done in python, which is easier for local development), there are multiple commands to start a CN from XML files, the code for starting a CN is always different (no one unified way to start "any" CN), Ping/Iperf require some support in ci_infra.yaml, ...

To help with the above, refactor the handling of core networks in the CI code as follows:

• define all CNs in ci_infra.yaml (basically how to deploy, undeploy, collect logs, and on which node)
• make one Python class that reads from ci_infra.yaml and that allows to start/stop previously defined CNs (and add unit tests...)
• make one XML command for start/stop of CNs, leveraging above class
• leverage the CNs defined in ci_infra.yaml consistently in Ping()/Iperf()
• delete old code

Furthermore, the following supporting changes/bugfixes are done:

• repair Iperf2_Unidir() and add unit test
• fix a number of invalid escape sequence errors (there are some remaining, which I will do soon)
• various bug fixes
• remove unused XML steps