Node list: handle issues with ironic inspector more gracefully

[Julie Pichon]

When the Ironic Inspector service is not reachable, the nodes list is not displayed at all and no nodes operations are possible.

How to reproduce
----------------
1. Mangle the Ironic inspector URL in the config (see e.g. bug 1703596, where it was set to /ironic instead of /ironic-inspector)
2. Navigate to the Nodes Page

Expected result
---------------
3. I see my registered nodes. The Introspection status might be "Unknown" or not displayed. I can still perform Ironic-only operations on nodes (e.g. Import).

Actual result
-------------
3. The Nodes page loads and shows "0 Nodes." If I try to click on Register Nodes -> Add New, nothing happens.

Version
-------
openstack-tripleo-ui-7.1.1-0.20170710093545.938119c.el7.centos.noarch

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/499216

[OpenStack Infra (hudson-openstack)]

Change abandoned by Ana Krivokapic (<email address hidden>) on branch: master
Review: https://review.openstack.org/499216

[Ana Krivokapić (akrivoka)]

After the discussion in the review, we concluded that this was not worth fixing as all services need to be function in order for the UI to be operational.