Allow host specific aliases to be added to /etc/postfix/generic

[drn05r]

Currently /etc/postfix/generic is built to just have a single aliases to redirect root's mail to SOWN's monitoring email address. It would be good if the template that builds this could include additional user-defined aliases for a host. In particular monitor needs this to ensure email sent out by Icinga meets DMARC SPF rules. Where /etc/postfix/generic on this server has been manually modified for the time-being.

I am not sure whether a bespoke field can be added to Netbox to record additional aliases or whether there is a better way to do this. The complexity for monitor is that it sends emails using the FQDNs for both ecs.soton.ac.uk and sown.org.uk. (ECS for Icinga application emails and SOWN when used from the command line of the icinga user).