[SRU] Align octavia-dashboard with ubuntu openstack dashboard packages

[Impact]
The octavia-dashboard package was synced late from Debian in Cosmic but does not align with the other Ubuntu OpenStack dashboard packages.

There are 4 things that need to be fixed:
1) python 3 interpreter must be used to collect/compress static assets
2) chown of /var/lib/openstack-dashboard must use horizon:horizon
see similar for sahara-dashboard: https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/sahara-dashboard/tree/debian/python3-sahara-dashboard.postinst?h=stable/rocky
3) the dashboard modules only needs to be copied to /usr/lib/python3/dist-packages/openstack_dashboard/enabled since /usr/share/openstack-dashboard/openstack_dashboard is a symlink in ubuntu
see similar for sahara-dashboard: https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/sahara-dashboard/tree/debian/rules?h=stable/rocky#n41
see alternatives postinst for horizon: https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/horizon/tree/debian/python3-django-horizon.postinst?h=stable/rocky#n6
4) debian/watch file should get release artifact from tarballs.openstack.org

[Test Case]
Deploy openstack, install octavia-dashboard on horizon node, and manually verify that octavia's dashboard works.

[Regression Potential]
The initial version is only in cosmic-proposed and didn't make it all of the way to cosmic proper.