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/tree1024.py

@@ -9,10 +9,13 @@ and running sysctl -p. Check util/sysctl_addon.
 
 from mininet.cli import CLI
 from mininet.log import setLogLevel
-from mininet.node import OVSSwitch
+from mininet.node import OVSSwitch, Host
 from mininet.topolib import TreeNet
+from mininet.examples.treeping64 import HostV4, QuietController
 
 if __name__ == '__main__':
     setLogLevel( 'info' )
-    network = TreeNet( depth=2, fanout=32, switch=OVSSwitch )
+    network = TreeNet( depth=2, fanout=32, host=HostV4,
+                       switch=OVSSwitch, controller=QuietController,
+                       waitConnected=True)
     network.run( CLI, network )