allow users to override the python package name to install

allow users to override the python package names. This is important with AWS Opsworks, the package `python` installs python 2.6, if you want to install python 2.7 you have to use the package named `python27`

All you have to do now to override the installed package is set the attribute to whatever you want

default['python']['python_pkgs']      = ["python27", "python27-devel"]