ovn-source option should only apply to ovn packages not to the whole system

[Gábor Mészáros]

When specifying ovn-source, the charm adds the source to /etc/apt/sources list file, but it should also provide pinning for the packages required to install and upgrade the ovn specific packages.
Currently if you have an openstack installed on the machine and deploy ovn-chassis with ovn-source specified (ie. openstack should come from distro and ovn from a specific release), the whole system will get upgraded, potentially breaking the openstack service.
A potential way to work the problem around, one can create /etc/apt/preferences configuration with something like:
Package: *
Pin: release n=jammy
Pin-Priority: 1000
Package: *ovn* *openvswitch*
Pin: release n=jammy-updates/zed
Pin-Priority: 1000

so that all packages come from distro and only the ovn/openvswitch ones are from the UCA.