Adding environment override attribute

Added some code to allow cookbook users override the default python/pip environment.
Was required to allow specifying http_proxy/https_proxy environment variables when using the pip/virtualenv install recipes and also to allow these environment variable to persist when using the python_pip LWRP

Example usage:
node.set['python']['environment'] = { :http_proxy => "http://...", :https_proxy => "https://..."}
include_recipe 'python'