Silence NC complaints.

As networkcache is really optional, silence it in case if not enabled nor not
availble.

src/zc/buildout/buildout.py

@@ -337,7 +337,7 @@ class Buildout(UserDict.DictMixin):
         os.chdir(options['directory'])
 
         if not LIBNETWORKCACHE_ENABLED:
-            self._logger.warning('Networkache disabled, no '
+            self._logger.debug('Networkache disabled, no '
                 'slapos.libnetworkache available.')
             self.download_cache_url = None
             self.download_dir_url = None
@@ -367,7 +367,7 @@ class Buildout(UserDict.DictMixin):
                 options['__networkcache__upload-cache-url'] = self.upload_cache_url
                 options['__networkcache__upload-dir-url'] = self.upload_dir_url
             else:
-                self._logger.info('Networkcache functionality not enabled. '
+                self._logger.debug('Networkcache functionality not enabled. '
                     'In order to activate use networkcache-section.')
                 self.download_cache_url = None
                 self.download_dir_url = None

src/zc/buildout/networkcache.txt

@@ -13,8 +13,6 @@ is shown:
    ... parts =
    ... ''')
    >>> print system(buildout)
-   Networkcache functionality not enabled. In order to activate use networkcache-section.
-   <BLANKLINE>
 
 But whenever networkcache configuration is enabled, it is possible to use it: