Nixpkgs: AMI release for 18.09

[roberth]

Issue description

We don't seem to have AMIs for NixOS 18.09. Have these been released?

• [x] Upload AMIs
• [x] Add AMI's to master branch nixos/modules/virtualisation/ec2-amis.nix
• [x] Backport to release-18.09

Steps to reproduce

Have a look at nixpkgs master

This solves out of memory errors during copy-closure during deployment.

[roberth]

@vcunat do you think this should be added to the release process?

[vcunat]

Possibly, I don't even know how AMIs work. So far these didn't seem to be "officially advertised". Also /cc @samueldr (18.09 co-manager).

I see AMI updates have been done mainly by @edolstra, but I suspect he might not be interested in them anymore.

[roberth]

Looks like they are "advertised".
They are also important for correct operation of NixOps on AWS.

[edolstra]

Sorry, didn't see this before. I'll generate new AMIs.

[c0bw3b]

@roberth 47dfe25e1b17cd405b81052ffcdab6ca92b467fa should have taken care of this
+ website is up-to-date

[vcunat]

And b5380ae736 on 18.09. Thanks everyone!