update documentation on formgrader configuration

[anaderi]

http://jupyterhub-deploy-teaching.readthedocs.io/en/latest/configure-nbgrader.html needs to be updated, since there is no deploy_formgrade.yml anymore and everything is configured through host_vars/<fqdn> file

PS I also think deploy_formgrade.yml was not completely useless, since configuration is essentially trial-and-error, and running it through deploy.yml would incur too much waiting.

[danroliver]

Also:
docs/source/repo_contents.md
docs/source/configure-nbgrader.rst
and
README.md

all mention deploy_formgrade.yml