Use the configFiles input argument to PyWPS

This pull request makes a PyWPS instance actually use the configFiles input argument to its __init__ method.

The input argument was already there, but was not being used in the code. I just wired it to the appropriate method.
I am finding this functionality useful in a apache mod_wsgi context. It allows me to control where the PyWPS instance will read its configuration inside the wpswsgi script , thus removing the need to setup an environment variable for PYWPS_CFG in the apache configuration file.