travis: force install of python2 into $PATH on macOS

travis: force install of python2 into $PATH on macOS

The homebrew formula's ignored Python PEP-0394 recommendations and
changed the plain python binary in /usr/local/bin to point to Python 3
instead of Python 2. Python 2 is not even installed into a location that
is in $PATH by default anymore. The homebrew packages print a message
to stderr claiming to provide a way to fix this

[quote]
  This formula installs a python2 executable to /usr/local/opt/python@2/bin

  If you wish to have this formula's python executable in your PATH then add

  the following to ~/.bash_profile:

    export PATH="/usr/local/opt/python@2/libexec/bin:$PATH"
[/quote]

When trying to update $PATH are suggested we find out this message is a
lie and /usr/local/opt/python@2 does not even exist, instead Python
seems to end up in /usr/local/Cellar/python@2/2.7.14_1

Rather than hardcoding this version specific directory in our travis
config, we change to run "brew link --force python@2", to make it create
symlinks in /usr/local/bin for the python2 binary.

The original change triggering this problem was

  https://github.com/Homebrew/homebrew-core/pull/24604#issue-171653084

There are countless bug reports against homebrew-core that are closed
without fixes, so it seems they are determined to ignore the Python
PEP 0394 recommendations on this.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>