libnetworkcache: make verification method public

Move `_verifySignatureInCertificateList` to a public method

slapos/libnetworkcache.py

@@ -421,7 +421,7 @@ class NetworkcacheClient(object):
       raise NetworkcacheException('Failed to parse json response (%r)' % data)
     if filter:
       return (data for data in data_list
-                   if self._verifySignatureInCertificateList(*data))
+                   if self.verifySignatureInCertificateList(*data))
     return data_list
 
   def _getSignatureString(self, content):
@@ -433,7 +433,7 @@ class NetworkcacheClient(object):
         return ''
     return b64encode(crypto.sign(k, content, 'sha1')).decode()
 
-  def _verifySignatureInCertificateList(self, content, signature_string):
+  def verifySignatureInCertificateList(self, content, signature_string):
     """
       Returns true if it can find any valid certificate or false if it does not
       find any.