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/test/test_clusterSanity.py

@@ -14,6 +14,8 @@ class clusterSanityCheck( unittest.TestCase ):
     def testClusterPingAll( self ):
         p = pexpect.spawn( 'python -m mininet.examples.clusterSanity' )
         p.expect( self.prompt )
+        p.sendline( 'py net.waitConnected()' )
+        p.expect( self.prompt )
         p.sendline( 'pingall' )
         p.expect ( '(\d+)% dropped' )
         percent = int( p.match.group( 1 ) ) if p.match else -1

examples/test/test_sshd.py

@@ -16,7 +16,8 @@ class testSSHD( unittest.TestCase ):
         "Log into ssh server, check banner, then exit"
         # Note: this test will fail if "Welcome" is not in the sshd banner
         # and '#'' or '$'' are not in the prompt
-        p = pexpect.spawn( 'ssh -i /tmp/ssh/test_rsa %s' % ip, timeout=10 )
+        ssh = 'ssh -o StrictHostKeyChecking=no -i /tmp/ssh/test_rsa ' + ip
+        p = pexpect.spawn( ssh, timeout=5 )
         while True:
             index = p.expect( self.opts )
             if index == 0:
@@ -57,4 +58,3 @@ class testSSHD( unittest.TestCase ):
 
 if __name__ == '__main__':
     unittest.main()
-