Currently, the sandbox provisioning from master branch fails at [this ansible task](https://github.com/edx/configuration/blob/master/playbooks/roles/nginx/tasks/main.yml#L278-L288) due to prospectus preview variables `PROSPECTUS_PREVIEW_HTPASSWD_USER` and `PROSPECTUS_PREVIEW_HTPASSWD_PASS`, not being defined by default.
This PR adds a default value to these variables which can be overridden if needed.
*JIRA tickets:* TBD
*Merge deadline:* ASAP - this is breaking provisioning that was previously working
[ ] @xitij2000
[ ] edX reviewer[s] TBD