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/popen.py

@@ -5,19 +5,15 @@ This example monitors a number of hosts using host.popen() and
 pmonitor()
 """
 
-
 from mininet.net import Mininet
-from mininet.node import CPULimitedHost
 from mininet.topo import SingleSwitchTopo
 from mininet.log import setLogLevel, info
-from mininet.util import custom, pmonitor
+from mininet.util import pmonitor
 
-def monitorhosts( hosts=5, sched='cfs' ):
+def monitorhosts( hosts=5 ):
     "Start a bunch of pings and monitor them using popen"
     mytopo = SingleSwitchTopo( hosts )
-    cpu = .5 / hosts
-    myhost = custom( CPULimitedHost, cpu=cpu, sched=sched )
-    net = Mininet( topo=mytopo, host=myhost )
+    net = Mininet( topo=mytopo, waitConnected=True )
     net.start()
     # Start a bunch of pings
     popens = {}