[deps] Fixed openssl dependency in Nginx Image #318

[pandafy]

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.

Description of Changes

In #318, I initially removed the OpenSSL dependency from the Nginx image, assuming it was included in the base image. However, the base image removes the OpenSSL package in a later build step. This commit reinstates the OpenSSL dependency to ensure proper functionality.

Related to #318

[pandafy]

I investigated why this issue was not caught by the CI and discovered that the openssl command is only triggered when the SSL certificate and key are missing. In our CI process, we first deploy the existing edge version before deploying the image built in the CI. As a result, the certificates were already present in the second deployment, so the openssl command was never executed, allowing the bug to go unnoticed.