Update configure.py to also link external_vars.yml

Like the server.pem key, there is an external_vars.yml file that contains
private information (passwords, private keys, git deploy keys, etc.) that
should not be placed in the public repository.

Each developer will need to have two resources thare are *never* checked into the repo
1. Their pem key
2. Their external_vars.yml file
Merge pull request #13 from glenjarvis/feature/add_external_vars_configuration_r0

Update configure.py to also link external_vars.yml
I've confirmed the merge without looking at the code (for the sake of expediency.)