container backend missing most options from NixOS

[bricewge]

At the moment the configuration options containers.* from NixOS aren't available in nixops. It makes this backend quite useless since most of the time you will want to access your container from outside (forwardPorts) or mount part of the host filesystem in there (bindMounts).

It's also talked about here: https://youtu.be/w1radFbG1GU?t=36m45s and related with #744.

A workaround is to use the none backed to directly configure the host instead of on a container basis.

[bricewge]

I don't use NixOs anymore. Please reopen this issue if you find it relevant still.