Nexus: Improve doc & debug code for ironic implementation

[Carol Bouchard]

Improve Nexus documentation and debug code with respect to ironic.
Peer was having issues with Ironic 'vpc_pool' where channel-group wasn't getting removed from the interface.
Could use some debug code in _delete_port_channel_resources to help triage this in the future.
Should also document the following:

[[post-config|/etc/neutron/plugins/ml2/ml2_conf.ini]]
#-baoli
[DEFAULT]
service_plugins = trunk,neutron.services.l3_router.l3_router_plugin.L3RouterPlugin
dns_domain = harmony.com.
#-baoli

[ml2]
#-baoli
extension_drivers = dns
#-baoli

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/609124

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/609124
Committed: https://git.openstack.org/cgit/openstack/networking-cisco/commit/?id=7d26ede72a28b9998d797d89ba7ea725d2fc3f83
Submitter: Zuul
Branch: master

commit 7d26ede72a28b9998d797d89ba7ea725d2fc3f83
Author: Carol Bouchard <email address hidden>
Date: Tue Oct 9 16:30:06 2018 -0400

    Improve Nexus Ironic related doc & logs

    Improve Nexus documentation and debug code with respect to ironic.
    Add Nexus documentation specifically for baremetal config.
    Could use some debug code in _delete_port_channel_resources to help
    triage issues in the future. Also document the need to configure
    dns for use with automated port-channels with baremetal.

    Change-Id: I21891526e09f6f40d8f8eb3715084563a2a02f80
    Closes-bug: #1795419