Update opsctl notify release for CAPV and CAPVCD

[yulianedyalkova]

Currently opsctl notify release is not implemented to deal with CAPV and CAPVCD clusters (as we didn't use releases there before).

For CAPV it returns an empty list:

opsctl notify release --file announcement.md --customers --provider vsphere --md-to-slack

For cloud-director it returns one customer.

[anvddriesch]

I looked at the command and it is getting the customers and their slack channels based on the information in the installations repo. (it is the same as what you see in opsctl list installations
It technically works just fine for CAPV and CAPVCD and shows the one customer who maintains an MC in CAPVCD. The others don't appear because while they use the releases, their MCs are on other providers and therefore labelled differently in the installations repo.
As it is we can't simply add another provider to an installation or at least we should make a conscious decision on how we should do it. Let me bring it to sig arch

[anvddriesch]

We decided to add an optional additionalProviders field in the installations repo which can contain a list of other providers than the primary provider.
We will also adjust notify release to respect this field.