libnetworkcache: make verification method public

Move `_verifySignatureInCertificateList` to a public method
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passed SlapOS.Eggs.UnitTest-Master.Python2

00:32:27

passed SlapOS.Eggs.UnitTest-Master.Python3

01:12:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:54:09