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/cpu.py

@@ -52,7 +52,7 @@ def bwtest( cpuLimits, period_us=100000, seconds=10 ):
                            period_us=period_us,
                            cpu=.5*cpu )
             try:
-                net = Mininet( topo=topo, host=host )
+                net = Mininet( topo=topo, host=host, waitConnected=True )
             # pylint: disable=bare-except
             except:
                 info( '*** Skipping scheduler %s and cleaning up\n' % sched )