Add support for fullchain-privkey.pem file creation and update

[lgunsch]

This adds a step to create a fullchain-priv.pem PEM file for each domain, which can then be used with Pound (load balancer and SSL termination).

It also updates the fullchain-priv.pem file every time the domains are renewed.

[puneetk]

for readability can you make this an actual file sourced in.

[lgunsch]

@puneetk Okay, I have made this a templated file now and sourced it in.

[iggy]

What kind of testing was done on this? (late to the game, I know)

[lgunsch]

@iggy I fully tested it on a server of mine with a live domain that previously didn't have a certificate. I tested that the fullchain-privkey.pem could be properly used by Pound. I also tested the bash scripts on another server with 5 real domains, but it is not managed by Salt at the moment.

[iggy]

Thanks.

[emansom]

Certbot supports post deployment hooks via the /etc/letsencrypt/renewal-hooks directory. Might this be moved there?
That way no reliance on cron nessecary.

Useful for masterless setups, that now might get a broken cert after some time, if they are relying on the fullchain-privkey.pem.