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 ;-(

mininet/net.py

@@ -173,7 +173,7 @@ class Mininet( object ):
         if topo and build:
             self.build()
 
-    def waitConnected( self, timeout=None, delay=.5 ):
+    def waitConnected( self, timeout=5, delay=.5 ):
         """wait for each switch to connect to a controller,
            up to 5 seconds
            timeout: time to wait, or None to wait indefinitely