librecipe: better implementation of getValueFromPreviousRun using buildout API

The API call is made on a function existing both in buildout 1.7 and buildout 2.X

slapos/recipe/librecipe/generic.py

@@ -26,7 +26,6 @@
 # Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 #
 ##############################################################################
-import ConfigParser
 import io
 import logging
 import os
@@ -287,12 +286,7 @@ class GenericBaseRecipe(object):
     Returns the value of a parameter from a previous run, if it exists.
     Otherwise, returns None
     """
-    if os.path.exists(self.buildout['buildout']['installed']):
-      with open(self.buildout['buildout']['installed']) as config_file:
-        try:
-          parser = ConfigParser.RawConfigParser()
-          parser.readfp(config_file)
-          return parser.get(section, parameter)
-        except:
-          pass
-    return None
\ No newline at end of file
+    try:
+      return self.buildout._read_installed_part_options()[section].get(parameter, None)
+    except KeyError:
+      return None
\ No newline at end of file