+ text file updates

This means that 'mn' will run be default in python3.

For python2, run

    sudo python2 `which mn`

Also:

- change mn execution line to /usr/bin/python
  (install seems to change it to python2 or python3)

- don't install python-is-python3 package

Also tweak README.md, INSTALL, and CONTRIBUTORS

thanks to Vintas Avinash
closes #1011

- call getLogger( "mininet" ) to set lg properly
- support warning() as well as (deprecated) warn()
- rearrange initialization slightly

Sometimes we got no output with python 3 on ubuntu 20
This simplifies setLogLevel slightly and should work
on python 2 and python 3.

fixes #1014 

Thanks to travis.org for the CI support for many years.

Should be able to build 16.04, 18.04 and 20.04,
as well as branch builds.

test_controlnet: log to stdout for now
test_walkthrough: wait for http server connection

In certain cases, dp0 and its interfaces were not
being cleaned up, probably due to scratchnet.py being
killed before it terminated. This in turn caused the
natnet test to fail.

also remove unneeded rm -f pexpect.out since we are not
generating that at the moment

Adjusting the tolerance for this test to compensate
for apparent performance variation when running as
part of github actions CI builds/tests.

* 2.3.0a1

* intfoptions and test_intfoptions tweaks

also added cleanup() to test_walkthrough fixture

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 ;-(

* Install correctly on Ubuntu 20.04

Main changes are to avoid pulling in python2 by mistake, and then to make sure to install only python3 packages.

Also: Add net-tools which is needed to get ifconfig on newer Ubuntu releases.

(admin note: there are still a couple of remaining 20.04 issues but this is helpful)

Initial support for github actions CI builds

Fix simple typo: udpates -> updates

Fix install.sh on Ubuntu 19.10: substitute cgroup-bin with cgroup-too…

change to use 0o for octal numbers w/ file permissons for python2/3 compatibility 

Fix usage of poll in mininet.utils.errRun