[SRU] Update package to adhere to new openstack-pkg-tools debconf templating

[Corey Bryant]

[Impact]
The magnum package was originally built with an older version of openstack-pkg-tools. When it is rebuilt with the current version of openstack-pkg-tools that is in bionic, the postinst script fails. This is due to consolidation of debconf templating within openstack-pkg-tools.

For example the magnum-common.postinst script fails with:

root@b1:~# DEBCONF_DEBUG=developer exec /usr/share/debconf/frontend /var/lib/dpkg/info/magnum-common.postinst configure
debconf (developer): frontend started
debconf (developer): frontend running, package name is magnum-common
debconf (developer): starting /var/lib/dpkg/info/magnum-common.config configure
debconf (developer): <-- INPUT high magnum/configure_db
debconf (developer): --> 30 question skipped
debconf (developer): <-- GO
debconf (developer): --> 0 ok
debconf (developer): <-- GET magnum/configure_db
debconf (developer): --> 0 false
debconf (developer): <-- INPUT high magnum/configure_rabbit
debconf (developer): --> 10 "magnum/configure_rabbit" doesn't exist
debconf (developer): <-- GO
debconf (developer): --> 0 ok
debconf (developer): <-- GET magnum/configure_rabbit
debconf (developer): --> 10 magnum/configure_rabbit doesn't exist

[Test Case]
sudo DEBIAN_FRONTEND=noninteractive apt install magnum-api magnum-conductor

[Regression Potential]
The changes are cherry-picked from the cosmic/rocky version of the package which limits the regression potential.

[Discussion]

[Robie Basak (racb)]

An upload of magnum to bionic-proposed has been rejected from the upload queue for the following reason: "Cleaning up the queue as https://bugs.launchpad.net/ubuntu/+source/magnum/+bug/1789425/comments/5 remains unanswered.".