Support reassigning of env variables in che-plugin.yaml

[metlos]

Description

Because we want to support using official images for tooling like Go compiler, etc, we need to have a way of mapping the way we configure the plugin container using Che specific env vars to the env vars that the tool itself expects.

E.g. we should make the following possible in che-plugin.yaml:

env:
- name: GOPATH
  value: "$(CHE_PROJECTS_ROOT)"

where CHE_PROJECTS_ROOT is the env var we expose to the plugin container that contains the mount path of the volume with the project files.