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/treeping64.py

@@ -4,21 +4,34 @@
 
 
 from mininet.log import setLogLevel, info
-from mininet.node import UserSwitch, OVSKernelSwitch  # , KernelSwitch
+from mininet.node import UserSwitch, OVSKernelSwitch, Host, Controller
 from mininet.topolib import TreeNet
 
+
+class HostV4( Host ):
+    "Try to IPv6 and its awful neighbor discovery"
+    def __init__( self, *args, **kwargs ):
+        super( HostV4, self ).__init__( *args, **kwargs )
+        cfgs = [ 'all.disable_ipv6=1', 'default.disable_ipv6=1',
+                 'default.autoconf=0', 'lo.autoconf=0' ]
+        for cfg in cfgs:
+            self.cmd( 'sysctl -w net.ipv6.conf.' + cfg )
+
+
+
+
 def treePing64():
     "Run ping test on 64-node tree networks."
 
     results = {}
-    switches = {  # 'reference kernel': KernelSwitch,
-                  'reference user': UserSwitch,
-                  'Open vSwitch kernel': OVSKernelSwitch }
+    switches = { 'reference user': UserSwitch,
+                 'Open vSwitch kernel': OVSKernelSwitch }
 
     for name in switches:
         info( "*** Testing", name, "datapath\n" )
         switch = switches[ name ]
-        network = TreeNet( depth=2, fanout=8, switch=switch )
+        network = TreeNet( depth=2, fanout=8, switch=switch,
+                           waitConnected=True )
         result = network.run( network.pingAll )
         results[ name ] = result