[Rdo-list] rdo-manager hostname issues with rabbitmq

[James Slagle]

Hi, there were a few different threads related to rdo-manager and encountering
rabbitmq errors related to the hostname when installing the undercloud. 

I was able to reproduce a couple different issues, and in my environment it
came down to $HOSTNAME not matching the defined FQDN hostname. You could
use hostnamectl to set a hostname, but that does not update $HOSTNAME in your
current shell.

rabbitmq-env, which is sourced at the start of rabbitmq-server, reads the
hostname from $HOSTNAME in some scenarios, and then uses that value to define
the rabbit node name. Therefore, if you have a mismatch between $HOSTNAME and
the actual FQDN, things can go off the rails with rabbitmq.

I've tried to address this issue in this patch:
https://review.gerrithub.io/#/c/232052/

For the virt-setup, setting of the FQDN hostname and adding it to /etc/hosts
will now be done automatically, with the instack.localdomain used. The hostname
can always be redefined later if desired.

For baremetal, I've added some notes to the docs to hopefully cover the
requirements.

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-- James Slagle
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