`matrix_nginx_proxy_data_path` has always served as a base path,
so we're renaming it to reflect that.
Along with this, we're also introducing a new "data path" variable
(`matrix_nginx_proxy_data_path`), which is really a data path this time.
It's used for storing additional, non-configuration, files related to
matrix-nginx-proxy.
## (BC break) matrix-nginx-proxy data variable renamed
`matrix_nginx_proxy_data_path` was renamed to `matrix_nginx_proxy_base_path`.
There's a new `matrix_nginx_proxy_data_path` variable, which has a different use-purpose now (it's a subdirectory of `matrix_nginx_proxy_base_path` and is meant for storing various data files).