/etc/openstack-release file is not removed when the package is uninstalled

[Felipe Reyes]

The package openstack-release installs the file /etc/openstack-release which is meant to determine what version of openstack that machine is running.

From deb package perspective that package is marked as a configuration file which means when the package is removed the file stays around giving, possibly, stale information to the charm (primary consumer of said file), or if a sysadmin modifies the file the apt-get option used by the charm `-o Dpkg::Options::="--force-confold"` won't overwrite its content and leading to multiple issues when upgrading an openstack cloud.

This package follows the model of lsb_release, although I think this information should be installed in a file under /usr/share/openstack-release/, this is what distro-info does.