There is a need to add delete command to fuel config cli

[Alexandr Kostrikov]

To fully support CRUD operations, we need to add delete sub-command to `fuel2 config set` `fuel2 config override` to delete set and overriden values.

[Alexandr Kostrikov (akostrikov-mirantis)]

Deletion should support keys:
level, key and env.

Like in set cmd:
fuel2 config set --env {env_id} --resource ' \
                      '{res_id} --value \'{{"a": 1, "b": null}}\' ' \
                      '--key key --type json ' \
                      '--level nodes=1

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/368881
Committed: https://git.openstack.org/cgit/openstack/tuning-box/commit/?id=db53fcb18785e4ab7fcd26ef232454d13f32e918
Submitter: Jenkins
Branch: master

commit db53fcb18785e4ab7fcd26ef232454d13f32e918
Author: Alexander Kislitsky <email address hidden>
Date: Mon Sep 12 17:38:44 2016 +0300

    Delete keys from resource values/overrides handled in fuel2

    Keys deletion for values/overrides handled.
    Default cliff formatter used for values/overrides operations.
    Message on values/overrides set command added.

    Change-Id: Ie47ff6a1cb98764447d045816ea6472a0701dcc3
    Closes-Bug: #1622600

[Tatyana Kuterina (tkuterina)]

Verified on 9.1 snapshot #280 with latest tuning-box