BUG: ExtensionsWizard: Fix CERTIFICATE_VERIFY_FAILED publishing error

This commit fixes SSL a regression introduced when updating the
version of python in r24526 (ENH: Upgrade to Python 2.7.10) because
of python/cpython@ae5b953 (pep 476: verify certificates by default (#22417))

The error was first reported in [1] and happened when contributing
an extension using the steps report below.

[1] http://slicer-devel.65872.n3.nabble.com/Problem-with-the-ExtensionWizard-td4035337.html

This commit fixes the issue by setting the SSL_CERT_FILE as suggested in
PEP-0476. See https://www.python.org/dev/peps/pep-0476/

Step to reproduce the problem:

// ----------------
$ bin/slicerExtensionWizard --create Slicer-MyTestExtension --addModule cli:MyTestCLI ~
copy template
'/home/jcfr/Projects/Slicer/Utilities/Templates/Extensions/Default' to
'/home/jcfr/Slicer-MyTestExtension', replacing 'TemplateKey' -> 'Slicer-
MyTestExtension'
[...]
created module 'MyTestCLI'

$ bin/slicerExtensionWizard --publish ~/Slicer-MyTestExtension/
Creating initial commit containing the following files:
MyTestCLI/Testing/Cxx/CMakeLists.txt
[...]
Continue y,n? y
creating github repository
Username for 'https://github.com': jcfr
Password for 'https://jcfr@github.com':
failed to publish extension: [SSL: CERTIFICATE_VERIFY_FAILED] certificate
verify failed (_ssl.c:590)
// ----------------

Reported-by: Jean-Baptiste Vimort <jvimort@umich.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24652 3bd1e089-480b-0410-8dfb-8563597acbee