EVEREST-1511 | Allow adding operators to existing DB namespaces

[mayankshah1607]

• Earlier it was not possible to add more operator installations on existing DB namespaces. This is because the installplan hook ran only once, during installation (i.e, in the post-install step)
• Since you'd have to run helm update .. to modify an existing helm release (to add more operator installations), this PR updates the chart hook to also run on post-upgrade
• Adds an additional check to not handle the InstallPlans of installed Subscriptions
• Also uses a more generic name for the chart hook
• Revert the operatorgroup name - use the one originally used in Everest, otherwise upgrading on existing installation breaks