Fix pip on custom dirs

1. updates get-pip.py (i don't know if this is actually necessary)
2. cleans up attributes to all use `node['python']`
3. fix pip recipe, when you have an alternate non-standard prefix (cwd doesn't get applied to the statement)
  - http://docs.opscode.com/resource_execute.html
```
When using the not_if and only_if guards with the execute resource, the current working directory attribute (cwd) is not inherited from the resource. For example:
execute 'bundle install' do
  cwd '/myapp'
  not_if 'bundle check' # This is not run inside /myapp
end
```