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/limit.py

@@ -34,7 +34,7 @@ def limit( bw=10, cpu=.1 ):
                       'Skipping this test\n' )
                 continue
         host = custom( CPULimitedHost, sched=sched, cpu=cpu )
-        net = Mininet( topo=myTopo, intf=intf, host=host )
+        net = Mininet( topo=myTopo, intf=intf, host=host, waitConnected=True )
         net.start()
         testLinkLimit( net, bw=bw )
         net.runCpuLimitTest( cpu=cpu )
@@ -43,7 +43,7 @@ def limit( bw=10, cpu=.1 ):
 def verySimpleLimit( bw=150 ):
     "Absurdly simple limiting test"
     intf = custom( TCIntf, bw=bw )
-    net = Mininet( intf=intf )
+    net = Mininet( intf=intf, waitConnected=True )
     h1, h2 = net.addHost( 'h1' ), net.addHost( 'h2' )
     net.addLink( h1, h2 )
     net.start()