Keep ansible-playbook from bombing without extra-vars

A recent change to ansible-playbook to support json extra-vars
also inadvertently broke the script when no extra-vars are
provided. Simply checking to make sure it is defined and truthy
should take care of the issue.
Merge pull request #3089 from kentfrazier/unsafe-extra-vars-fix

Keep ansible-playbook from bombing without extra-vars
Allow the group_vars and host_vars directories to be stored alongside the playbook as well as inventory.
Document group_vars and host_vars additional paths.
Warn when there is a space in the mount module opts parameter.
Update docs about examples, add edX online's configuration repo to examples
Update changelog
Standardize example
Standardize docs
Standardize docs
Standarize docs
Standardize docs
Standardize module doc
Module doc standardization/style
Docs/style
Docs/style
Docs standardization/style
Docs standardization/style
Doc standardization/style
Docs standardization