Change initial --generate-config step to use ACME or raise alarm bells about the certificate not working

[turt2live]

A config file has been generated in 'homeserver.yaml' for server name 'travisr.com' with corresponding SSL keys and self-signed certificates. Please review this file and customise it to your needs.
If this server name is incorrect, you will need to regenerate the SSL certificates

This should be louder and say that the certificate needs to be replaced.

[richvdh]

we should kill off --generate-config imho

[richvdh]

hopefully better as of #4625