build: Make networking work inside LXC builder in gitian-building.md

These are changes I needed to get gitian building to work with Debian
8.2, which is the version we tell to use.

- Set up NAT, so that container can access network beyond host
- Remove explicit cgroup setup - these are mounted automatically now
- gitian: Need `ca-certificates` and `python` for LXC builds

Github-Pull: #7060
Rebased-From: 99fda26de0 3b468a0e60

contrib/gitian-descriptors/gitian-osx.yml

@@ -18,6 +18,8 @@ packages:
 - "libcap-dev"
 - "libz-dev"
 - "libbz2-dev"
+- "ca-certificates"
+- "python"
 reference_datetime: "2016-01-01 00:00:00"
 remotes:
 - "url": "https://github.com/bitcoin/bitcoin.git"