Use the index in grubenv (#1209678)

Trying to exactly match the name that grub2 uses for the menutitle is
easier to do using index instead of trying to reconstruct the title
name.

Resolves: rhbz#1209678
(cherry picked from commit 15338de)

pyanaconda/bootloader.py

@@ -1584,8 +1584,14 @@ def write_config(self):
 
         # make sure the default entry is the OS we are installing
         if self.default is not None:
-            entry_title = "0"
-            rc = iutil.execInSysroot("grub2-set-default", [entry_title])
+            # find the index of the default image
+            try:
+                default_index = self.images.index(self.default)
+            except ValueError:
+                log.warn("Failed to find default image (%s), defaulting to 0", self.default.label)
+                default_index = 0
+
+            rc = iutil.execInSysroot("grub2-set-default", [str(default_index)])
             if rc:
                 log.error("failed to set default menu entry to %s", productName)