Support multiple credential types in DTRS

Currently site (default) and chef
Allow dot in valid identifiers
Call the core DTRS functions using the correct variable names

This makes it explicit which DTRS dashi calls keep using old variable
names for backward compatibility.
Fix typo in comment
DTRS and Provisioner support for Chef server ctx
Delete Chef node on terminate

Also improve error handling in node creation
Add pychef to setup.py and requirements.txt
Feed Chef credential name in via Provisioner vars
EPUM support for Chef credential name in domain

The value "chef_credential" is loaded from the domain general
config and passed through in provisioner vars for all VMs
launched by the domain.
Merge pull request #18 from ooici/dtrs_chef_credentials

Chef server contextualization support