[fix] Nginx config location admin/ allowed network list

[asmodehn]

Unroll the allowed network list in location admin/, as explicited in nginx docs: https://nginx.org/en/docs/http/ngx_http_access_module.html

Also adds another openwisp2_admin_allowed_networks variable to keep openwisp2_admin_allowed_network for backward compatibility.

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #518, #481.

Description of Changes

This loop unrolls the value of the variable, to pass nginx config parser

[nemesifier]

Thanks @asmodehn, I think openwisp2_admin_allowed_networks need a default value ([], empty list) and needs to be mentioned in the README (we can remove the old variable to encourage new users to use this instead of the old one).