support custom key names for bring your own certificate

[jpuskar]

Hi,
It looks like the init sidecar is hard-coded to use cert.pem and tls.pem. This prevents one from using cert-manager to handle webhook certificates, for those of us who can't use the built-in k8s certificate generation (e.g. canonical charmed kubernetes default installation).

Would you consider allowing for an environment variable that replaces the strings 'cert.pem' and 'tls.pem' with a user-provided value?

Cert-manager uses tls.key, tls.crt, and ca.crt by default.

[phenixblue]

I think this is something fairly simple we can do. I am returning from holiday and it will take me a bit to catch up on things, but I will try and take care of this as soon as possible.