Fix stderr parse into JSON for OS List opers

The list operation is looking to parse json, however stderr will include
warnings from the openstack client, like this one:
https://storyboard.openstack.org/#!/story/2003132

Therefore parsing to json will fail. This patch changes the List
operations to only parse stdout as json.

Change-Id: Ib6dc6b99f13b3288effc5e9c2e6c5cda4636d5a7
Signed-off-by: Tim Rozet <trozet@redhat.com>