Add 'fuel2 graph delete' command

[OpenStack Infra]

https://review.openstack.org/374326
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit d4140d54726cbcabde75acb856546026a5f40295
Author: tivaliy <email address hidden>
Date: Wed Sep 21 21:23:09 2016 +0300

    Add 'fuel2 graph delete' command

    Currently Nailgun supports DELETE deployment graph operation
    3d1f96e529c62b34877216411fc7545b84666d4b. However
    python-fuelclient doesn't implement this feature. This patch
    adds 'fuel2 graph delete' command to support deployment graph
    removal process:

     fuel2 graph delete [-h] (-e ENV | -r RELEASE | -p PLUGIN)
                        -t GRAPH_TYPE

    DocImpact
    Closes-Bug: 1624918

    Change-Id: If49d4f10d404edbd04bb9f1a976b36de1cb1dcfe