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 ;-(

.github/workflows/run-tests.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-16.04]
+        os: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04]
         python-version: [3.x, 2.x]
     steps:
       - name: Check out Mininet source
@@ -47,12 +47,10 @@ jobs:
           export sudo="sudo env PATH=$PATH"
           export PYTHON=`which python`
           rm -f pexpect.out
-          $sudo $PYTHON mininet/test/runner.py -v -quick || \
-            (cat pexpect.out && exit 1)
+          $sudo $PYTHON mininet/test/runner.py -v -quick
       - name: Run examples tests (quick)
         run: |
           export sudo="sudo env PATH=$PATH"
           export PYTHON=`which python`
           rm -f pexpect.out
-          $sudo $PYTHON examples/test/runner.py -v -quick || \
-            (cat pexpect.out && exit 1)
+          $sudo $PYTHON examples/test/runner.py -v -quick