Rename server.crt and server.key to tls.crt and tls.key

[jessesuen]

We should update the argocd-secret such that it uses the standard tls.crt and tls.key keys in the secret instead of its current keys: server.crt and server.key. This will make the argocd-secret compatible with things like Ingress and Cert Manager. Note that this will require the user to perform a manual step of copying or renaming the keys before the upgrade and needs to be documented as a breaking change in the release notes.

[jessesuen]

I confirmed that cert-manager will update an existing secret with just those two keys and not replace the entire secret:
cert-manager/cert-manager#221

[mrbobfrog]

I confirmed that cert-manager will update an existing secret with just those two keys and not replace the entire secret:
cert-manager/cert-manager#221

@jessesuen We may want to add that tidbit to the argocd docs to save the next person the trouble of tracking release notes to issues and comments... I can make a PR