Make graph download command more predictable

[OpenStack Infra]

https://review.openstack.org/392419
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 13acdfba3727728451640c77fa40538ca615949d
Author: tivaliy <email address hidden>
Date: Wed Nov 2 07:51:23 2016 +0200

    Make graph download command more predictable

    Now if we try to download graph with any task level parameters
    without specifying any filename the result (by default) will be
    stored in 'cluster_graph.yaml'.
    This patch:
      * makes default filename more accurate based on input parameters
        (environment id, task level graph value and format)
      * adds support to download graph in JSON format

      fuel2 graph download [-h] -e ENV (-a | -c | -p | -r)
                           [-t GRAPH_TYPE][-f FILE]
                           [--format {json,yaml}]

    DocImpact

    Change-Id: Ia7aa03cbbcde3d63e4ade5e76ea1242a2939b71e