When --burst-limit/$HELM_BURST_LIMIT is set, the specified value is not
currently used for the discovery client instantiated by
genericclioptions. This change sets genericclioptions.discoveryBurst to
the value of --burst-limit, meaning it should now be possible to fix
client-side throttling issues encountered by the discovery client.
This value is only configured if --burst-limit is actually set. If
--burst-limit is set to the default value, then discoveryBurst should be
left at its default of 300.
Closes#13128
Signed-off-by: Evan Foster <efoster@adobe.com>