[diagnostics] Use masterConfig in a field member of MasterConfigCheck struct

[nak3]

Although masterConfig was set in a filed of MasterConfigCheck at
complete():

origin/pkg/oc/admin/diagnostics/diagnostics/host/check_master_config.go

Line 38 in f3ca235

d.masterConfig = masterConfig

the set value is not used, but used from util.go's global value:

origin/pkg/oc/admin/diagnostics/diagnostics/host/check_master_config.go

Line 53 in f3ca235

results := configvalidation.ValidateMasterConfig(masterConfig, nil)

(though I think that there is no remarkable issue due to this at this
moment) this patch updates to use proper value.

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[openshift-bot]

@nak3: PR needs rebase.

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[nak3]

Closing as #19509 deleted the code.