Several fixes in Travis control file for OS-X
Several fixes in Travis control file for OS-X

Details:

* Optimized Python installation on OS-X; it is now only performed when the
  respective Python command cannot be found.

* Made the virtualenv creation on OS-X consistent with zhmc-ansible-modules
  repo.

* Fixed that on OS-X and with Python 3, the 'python3' and 'pip3' commands
  were used despite the fact that a virtualenv had been created. This has
  been changed to now always use 'python' and 'pip' commands.

Signed-off-by: Andreas Maier <38dbb04957d222ea6ce72de72413b1e55e665472@de.ibm.com>

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient