CI matrix build + 20.04 fixes (#990)

Ubuntu 20.04 fixes:
- fixes sshd test
- speeds up examples/{treeping64,tree1024}.py
- debugging hacks/output for testLinkChange
- removes cfs from examples/popen.py
- improves nat in nodelib (netplan fixes?)
- makes some tests executable
- waits for switches to connect in tests to
  avoid race conditions
-- adds mn -w option and wait CLI command
Changes:
- REMOVES default "-v" argument for Controller()
  and adds verbose(=False) option; avoiding logging
  makes it faster
- CHANGES waitConnected to wait for 5 seconds
  as documented; we may wish to implement an argument
  to -w to set this timeout
Issues?
- There may still be an issue with the ovs-netplan-clean
  service causing the boot to hang ;-(

README.md

@@ -132,4 +132,4 @@ Bob Lantz
 Mininet Core Team
 
 [1]: https://travis-ci.org/mininet/mininet.svg?branch=master
-[2]: https://github.com/mininet/mininet/workflows/mininet%20tests/badge.svg
+[2]: https://github.com/mininet/mininet/workflows/mininet-tests/badge.svg