fix: only add TLS config if cert file has been written to disk

[lucabello]

Issue

Fixes #506.

This likely happens when _configure() is called by something other than _on_server_cert_changed (e.g., a perfectly timed update-status), but when the certificate is already in relation data (or Prometheus has already been related through the certificates interface but the certificate is not ready yet).

Solution

Only add the tls_server_config for Prometheus if the certificate exists on the file sytstem.

The _configure() method is also called by _on_server_cert_changed(), which (sequentially) writes the cert to disk before calling _configure(). This means we don't risk skip adding the TLS configuration with this extra check.

Testing Instructions

It's pretty much a race condition, so it's hard to add a test for it. I guess trying to replicate the issue is something, though of course it doesn't guarantee it.

Release Notes

Only add the tls_server_config for Prometheus if the certificate exists on the file sytstem.

[lucabello]

I don't think it's necessarily in the scope of this issue, which is the failed replan; this behavior when TLS is enabled has been implemented in the TLS PR :)

I'll add it anyway!

[lucabello]

After noticing the itests were passing locally, rerunning them fixed things :D