[WorkInProgress] Add config for network plugin directory

[tomdee]

Supersedes #1950

[jonboulle]

/cc @krnowak

[krnowak]

It is more or less the step in the correct way, but there are several issues:

• I don't like the mixing of the configuration for stage0 and stage1. See config changes for stage0 and stage1 #2013 for ideas.
• Importing github.com/coreos/rkt/rkt will not work as intended, because
  • it is a "main" package, and importing "main" packages started to fail in go 1.5.
  • code in github.com/coreos/rkt/rkt largely belongs to the rkt binary, and the "networking/net_plugin.go" code is invoked in a different binary (init from the stage1 image), so init will not get the overridden configuration paths.

My plan is to split the rkt config stuff into general config stuff and then stage0 config and stage1 config.

[yifan-gu]

Re @krnowak
As #2013 is a breaking change, it probably won't get in very soon. However, without the ability to specify the net dir and plugin dir, it makes rktnetes a hard time to use the net config and plugins users provided to k8s. See https://github.com/kubernetes/kubernetes/blob/33ef7a93a40a361904ac68a252e92beed8d444ca/docs/admin/network-plugins.md#cni
So I wonder if we can get this in first for now?

[yifan-gu]

Sorry I misread the PR. This seems to enable us configure the dir for the net plugin binaries. But we also need the ability to configure the dir for the net configs.

[krnowak]

@yifan-gu: #2013 was described as breaking change, but that was before 1.0 was released. Right now, this change would be backward-compatible, but the old directories would be deprecated.