[RDY] Format domain XML from a file to allow for more flexibility

[teto]

Related to #883.

The PR adds a deployment.libvirtd.template setting so that users can upload their own domain.xml. They can use a few variables that will be replaced such as {interfaces} (look at domain.xml.in for an example).
I wonder if it wouldn't be best to accept a string instead of a filename ?

I need to test it a bit more.

[teto]

I changed the parameter to accept a string instead as it seems more "nix-like", having an extra file is a bother.
Eventually this setting could replace extraDevicesXML / extraDomainXML.

[teto]

Here I tried to replace the memory_size=defn.memory_size, by **defn (more elegant) but couldn't succeed.

[teto]

I want to use shared folders with libvirt (http://rabexc.org/posts/p9-setup-in-libvirt), if there is any interest I can provide a proper interface deployment.libvirtd.sharedFolders but this PR would allow me to make it work inbetween.

EDIT: I guess I can use extra_devices for the shared folders, but to adjust the XML to allow debug a kernel, the PR is still needed

[teto]

bump. This should be backwards compatible.

[teto]

@rbvermaa what do you think ?

[AmineChikhaoui]

I guess I'm confused what's the point of having this as a nixops option ?
What if someone changes this to:

deployment.libvirtd.template = ''
<other template that doesn't use the same variables>
''

That'll basically make it inconsistent with what the python code expects

[grahamc]

Hello!

Thank you for this PR.

In the past several months, some major changes have taken place in
NixOps:

1. Backends have been removed, preferring a plugin-based architecture.
   Here are some of them:

   • https://github.com/NixOS/nixops-aws
   • https://github.com/NixOS/nixops-hetzner
   • https://github.com/nix-community/nixops-vbox
   • https://github.com/nix-community/nixops-libvirtd
   • https://github.com/nix-community/nixops-datadog

2. NixOps Core has been updated to be Python 3 only, and at the
   same time, MyPy type hints have been added and are now strictly
   required during CI.

This is all accumulating in to what I hope will be a NixOps 2.0
release. There is a tracking issue for that:
#1242 . It is possible that
more core changes will be made to NixOps for this release, with a
focus on simplifying NixOps core and making it easier to use and work
on.

My hope is that by adding types and more thorough automated testing,
it will be easier for contributors to make improvements, and for
contributions like this one to merge in the future.

However, because of the major changes, it has become likely that this
PR cannot merge right now as it is. The backlog of now-unmergable PRs
makes it hard to see which ones are being kept up to date.

If you would like to see this merge, please bring it up to date with
master and reopen it. If the or mypy type checking fails, please
correct any issues and then reopen it. I will be looking primarily at
open PRs whose tests are all green.

Thank you again for the work you've done here, I am sorry to be
closing it now.

Graham