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(XXX: I realized while doing this that we don't support the multiplexed
link case in the linuxbridge case, I think.  Have to circle back and get
that someday...)

From the comments:

    My "Hm" was justified.  This is no fix; gnocchi cannot be run
    from python2 (it just feels like it can, until ceilometer
    starts trying to send it data).
    So we write out a quick systemd service and run gnocchi-api
    standalone (there is no service file installed with it).

If there are multiple vlan devs multiplexed atop one parent, we have to
append to an existing <parent-dev>.network file, not replace, so that
the parent file gets all the VLAN=<vlan-iface> settings it needs.

(Bridge port devices need a nearly empty .network file to be set UP; etc.)

Also needed a new service unit file (post network and pre testbed) to
handle the various file writes to testbed locations, that were
previously trivially handled as up hooks in /etc/network/interfaces.

https://bugs.launchpad.net/ubuntu/+source/gnocchi/+bug/1746992

https://bugs.launchpad.net/ubuntu/+source/gnocchi/+bug/1746992