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/linuxrouter.py

@@ -79,7 +79,8 @@ class NetworkTopo( Topo ):
 def run():
     "Test linux router"
     topo = NetworkTopo()
-    net = Mininet( topo=topo )  # controller is used by s1-s3
+    net = Mininet( topo=topo,
+                   waitConnected=True )  # controller is used by s1-s3
     net.start()
     info( '*** Routing Table on Router:\n' )
     info( net[ 'r0' ].cmd( 'route' ) )