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/clusterperf.py

@@ -8,7 +8,7 @@ from mininet.log import setLogLevel
 
 def perf(Link):
     "Test connectivity nand performance over Link"
-    net = Mininet( host=RemoteHost, link=Link )
+    net = Mininet( host=RemoteHost, link=Link, waitConnected=True )
     h1 = net.addHost( 'h1')
     h2 = net.addHost( 'h2', server='ubuntu2' )
     net.addLink( h1, h2 )