Werk #11632: Bulk discovery: Fix 'too many values to unpack' if old performance parameters are still used

Old bulk performance parameters in etc/check_mk/multisite.d/wato/global.mk may consists of three values: use-cache, do-scan, bulk-size. With some refactorings the first parameter was removed (which had no effect since Checkmk 1.5.0) and causes above error while executing bulk discovery.

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