Large number of realms causing slow startup due to new installation check

The check for new installation using applianceBootstrap.isNewInstall() at KeycloakApplication.migrateAndBootstrap() is causing all realms to be queried.

    public boolean isNewInstall() {
        if (session.realms().getRealms().size() > 0) {
            return false;
        }

Eventually this check will get slower and slower as the number of realms increase.

Checking if master realm exist will improve the performance.