Code review comment for ~tshinn/snap-store-proxy-charm:fix_https_cert_config

1. No, I treated snap-store-proxy itself as a black box while getting to this point. My focus was solely on fixing the charm. The charm didn't work as is because instead of using `import-certificate` it configured `proxy.tls.cert` and `proxy.tls.key` configurations directly, which didn't result in nginx getting configured properly (was unable to get to the bottom of exactly why). When I tried `import-certificate` it started working, hence the MP.

2. Yes I think you are right that the charm itself only supports a single certificate in the PEM file. When the snap-store-proxy supports it, the charm should be updated also! :)