Provide support for air gapped env for rally

Provide support for air gapped env for rally

Sometimes, tested Kubernetes doesn't have direct access to Internet but
access through repository mirrors.
This patch handles this case for rally test case.

There's two ways for providing the repository mirrors:

- Give an environment variable (`MIRROR_REPO`) which gives a repository
  mirro with all needed images.
- Gives an environment variable per needed repo:
  - `DOCKERHUB_REPO` for DockerHub repository (`docker.io`)
  - `GCR_REPO` for Google Cloud repository (`gcr.io`)
  - `K8S_GCR_REPO` for Kubernetes repository (`k8s.gcr.io`)

Needed images list has also been extracted so Kubernetes administrator can
easily upload these images to the mirror if the mirror also doesn't have
access to Internet.

Signed-off-by: Sylvain Desbureaux <sylvain.desbureaux@orange.com>
Change-Id: I2ea6622b79f7e3c3c63c1441c4dab48e9bc4fb1a