CI: Verify old channels are kept after an elemental-operator upgrade

[davidcassany]

The default channel name and resources are not expected to be constant over upgrades anymore. That is for v1.5.x series the default channel is elemental-channel and for the 1.6.x series the default channel is sl-micro-6.0-baremetal-channel. More over for upcoming versions the expectation is that there will be multiple channels (one per flavor) with a similar naming convention to: <base OS distro>-<flavor>-channel. During upgrades any former or pre existing default channel should not be removed, however on new installations only latest channels are included by default.

Would be nice to verify that any default channel that exists before upgrading is still available after upgrading. As an example after upgrading from v1.5 to v1.6 there should be two default channels available: elemental-channel and sl-micro-6.0-baremetal-channel.

I wonder if we could add a generic check, such as all default channels are still available after upgrading, so we do not depend on specific versions or naming conventions. We define default channels as resources that are managed by Helm, so that is any channel including the label app.kubernetes.io/managed-by: Helm, which should be all of them unless additional ones are included manually.