Also means we don't have to resort to an update statement to set parent
for child groups who also have clusters.
This is much shorter than
```
create(:cluster, :provided_by_gcp, :group, groups: [group])
```
- This enables us to use a scope to query all clusters in group
hierarchy order in one query, and also enables us to union to instance
clusters later.
- Handle case where clusters not present at level. In which case the
query should go ahead and return the next level's clusters.
- Swap with new CTE query behind Feature flag. This FF is default
disabled.
With this MR, group clusters is now functional, so default to enabled.
Have a single setting on the root ancestor group to enabled or disable
group clusters feature as a whole
Look for matching clusters starting from the closest ancestor, then go
up the ancestor tree.
Then use Ruby to get clusters for each group in order. Not that
efficient, considering we will doing up to `NUMBER_OF_ANCESTORS_ALLOWED`
number of queries, but it's a finite number
Explicitly order query by depth
This allows us to control ordering explicitly and also to reverse the
order which is useful to allow us to be consistent with
Clusters::Cluster.on_environment (EE) which does reverse ordering.
Puts querying group clusters behind Feature Flag. Just in case we have
issues with performance, we can easily disable this
