Trouble adding IPv6 to installed Keystone charm.

[Ken D'Ambrosio]

Hey, all.  Looking to enable IPv6 endpoints on our Newton cloud.  "juju 
config keystone" shows this:

  prefer-ipv6:
     description: |
       If True enables IPv6 support. The charm will expect network 
interfaces
       to be configured with an IPv6 address. If set to False (default) 
IPv4
       is expected.
       .
       NOTE: these charms do not currently support IPv6 privacy 
extension. In
       order for this charm to function correctly, the privacy extension 
must be
       disabled and a non-temporary address must be configured/available 
on
       your network interface.
     type: boolean
     value: true

It had originally been false.  I manually added 2004::100 to eth0 on my 
keystone host, and changed the value to true.  The Juju log on the 
Keystone host comes back with this:

2017-12-04 19:16:06 INFO config-changed Exception: Interface 'eth0' does 
not have a scope global non-temporary ipv6 address.

eth0 looks like this:
eth0      Link encap:Ethernet  HWaddr 00:16:3e:7f:eb:70
           inet addr:172.23.248.38  Bcast:172.23.248.63  
Mask:255.255.255.192
           inet6 addr: 2004::100/64 Scope:Global
[...]

I've googled like crazy, and can't find anything that really seems to 
fit the bill for "non-temporary ipv6 address", except in a different 
charm 
(https://api.jujucharms.com/v5/~sdn-charmers/trusty/contrail-analytics-31/archive/hooks/charmhelpers/contrib/network/ip.py) 
which says:

     We currently only support scope global IPv6 addresses i.e. 
non-temporary
     addresses. If no global IPv6 address is found, return the first one 
found
     in the ipv6 address list.

Which seems to imply that a global IPv6 address *is* non-temporary by 
definition, which confuses me even more as to what's broken.

Any pointers here?

Thanks kindly,

-Ken