Add ability to the `install_flux.sh` script to use existing registry secret if it exists.

Feature Request

Why

Currently the install_flux.sh requires a username/password to generate the secret in order to pull images from registry1. If that secret already exists, it'd be more streamlined to use it without having to look up the username/password every time when upgrading Flux.

The more detail here the better!

Proposed Solution

Add a --use-existing-secret flag to the install script to only conditionally create the secret. Potentially add logic that, if the flag is set, to confirm the secret actually exists and fail fast if it doesn't.