Closes #1003  

* rc.local: /usr/bin/bash -> /bin/bash

bash is (and should be) located in /bin

this may have caused ubuntu 18.04 to not execute
/etc/rc.local and regenerate ssh keys

* fix bootAndRun()

This helps with virtualenv although it can open
up another security hole if you end up using an
unexpected python interpreter.

Overall it seems to make sense to err on the side
of usability but it's good to be aware of security.

However, for the remaining utility scripts that require
python 2, we explicitly note this with #!/usr/bin/python2.

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

fixes #1014 

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

* 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)

Ubuntu 16.04 no longer includes a 'python' executable.

If none of {$PYTHON,python,python2,python3} are usable, we advise
the user to either set PYTHON or install a usable python.

Perhaps we should have an option for py2/p3, but for
now we detect the default python version and it can
also be specified via the PYTHON env var.

Changed netbee URL.
Changed libxerces package for Ubuntu 18.04

This should also be backward compatible to 14.04.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>

Fixes #713

An unfortunate side effect of switching from print to output() is
that all output() goes to stderr. We should probably carefully
consider whether this is the right thing to do.

This was well-intentioned, but it causes more trouble
than it's worth.

Fixes #652

We should probably do this in a more automatic way.

- add xenial
- partx returns an error (warn rather than fail)
- remove dnsmasq build.py dependency (not really needed, side effects)

The submodules have submodules with authenticated URLs which breaks anonymous,
automated builds. Excluding them also decreases build time.

git -C is not available in git 1.8 (CentOS 7)

Update command was hard-coded for debian or ubuntu based systems which was 'apt-get'. Setting an update variable accordingly solves this problem.

It's a bad idea for all Mininet VMs to share the same SSH keys.
Certainly users can regenerate their own keys, but it's better
if we don't ship a key and simply regenerate it on boot.

Keeping "use" for now even though it seems harder to remember
than "run". And the build/test options are probably much
clearer being inline in the help.