can not upload network yaml for a node group created post deployment

It's not possible to upload new network yaml to an environment that is already deployed. This prevents expanding of existing environment with new nodegroups forcing users to pre-create artificial nodegroups and corresponding networks.

VERSION:
  feature_groups:
    - mirantis
    - experimental
  production: "docker"
  release: "6.1"
  openstack_version: "2014.2.2-6.1"
  api: "1.0"
  build_number: "521"
  build_id: "2015-06-08_06-13-27"
  nailgun_sha: "4340d55c19029394cd5610b0e0f56d6cb8cb661b"
  python-fuelclient_sha: "4fc55db0265bbf39c369df398b9dc7d6469ba13b"
  astute_sha: "7766818f079881e2dbeedb34e1f67e517ed7d479"
  fuel-library_sha: "f43c2ae1af3b493ee0e7810eab7bb7b50c986c7d"
  fuel-ostf_sha: "7c938648a246e0311d05e2372ff43ef1eb2e2761"
  fuelmain_sha: "bcc909ffc5dd5156ba54cae348b6a07c1b607b24"

Steps to reproduce:
1) create new environment with neutron+gre
fuel env create --name nodegroup-test --release 2 --nst gre

2) create new node group in this environment
fuel --env 1 nodegroup --create --name "tor1"

3) download network yaml
fuel --env 1 network --download

4) modify the file reflecting new network addressing scheme

5) upload the file
fuel --env-1 network --upload

6) add new nodes to the environment

7) deploy changes

8) once finished create a new nodegroup
fuel --env 1 nodegroup --create --name "tor2"

This operation finishes successfully

9) download new network yaml
fuel --env 1 network --download

10) modify network details of newly created nodegroup

11) upload modified yaml
fuel --env-1 network --upload

Expected behavior:
New yaml is accepted, networks associated with newly created nodegroup are updated based on information from yaml so user can expand existing environment with compute nodes in new network segments

Current behavior:
Upload fails with the following error:
403 Client Error: Forbidden (Network configuration can't be changed after, or in deploy.)