Honor ansible_private_key_file for delegated hosts

If a delegated host is not found in the inventory specified
private_key_file for primary host was not used.

This allows running playbooks without having to define any inventory at
all and to use the same ssh private key for both primary host and
delegated one.
Merge branch 'delegate_to_honor_ssh_private_key_file' of https://github.com/renard/ansible into renard-delegate_to_honor_ssh_private_key_file
Commenting out pkg_resources lines for now, as they break hacking/env-setup