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/cluster.py

@@ -875,7 +875,7 @@ class MininetCluster( Mininet ):
 
 def testNsTunnels( remote='ubuntu2', link=RemoteGRELink ):
     "Test tunnels between nodes in namespaces"
-    net = Mininet( host=RemoteHost, link=link )
+    net = Mininet( host=RemoteHost, link=link, waitConnected=True )
     h1 = net.addHost( 'h1')
     h2 = net.addHost( 'h2', server=remote )
     net.addLink( h1, h2 )
@@ -891,7 +891,8 @@ def testNsTunnels( remote='ubuntu2', link=RemoteGRELink ):
 def testRemoteNet( remote='ubuntu2', link=RemoteGRELink ):
     "Test remote Node classes"
     info( '*** Remote Node Test\n' )
-    net = Mininet( host=RemoteHost, switch=RemoteOVSSwitch, link=link )
+    net = Mininet( host=RemoteHost, switch=RemoteOVSSwitch, link=link,
+                   waitConnected=True )
     c0 = net.addController( 'c0' )
     # Make sure controller knows its non-loopback address
     Intf( 'eth0', node=c0 ).updateIP()