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 ;-(

examples/sshd.py

@@ -29,7 +29,7 @@ from mininet.util import waitListening
 def TreeNet( depth=1, fanout=2, **kwargs ):
     "Convenience function for creating tree networks."
     topo = TreeTopo( depth, fanout )
-    return Mininet( topo, **kwargs )
+    return Mininet( topo, waitConnected=True, **kwargs )
 
 def connectToRootNS( network, switch, ip, routes ):
     """Connect hosts to root namespace via switch. Starts network.