Asset Fetcher: use hash_file name for the opened hash file

And not "asset_file", because that references the file with the
content itself, and not the file containing its hash.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/asset.py

@@ -192,8 +192,8 @@ class Asset(object):
         if not os.path.isfile(self.hashfile):
             self._compute_hash()
 
-        with open(self.hashfile, 'r') as asset_file:
-            for line in asset_file:
+        with open(self.hashfile, 'r') as hash_file:
+            for line in hash_file:
                 # md5 is 32 chars big and sha512 is 128 chars big.
                 # others supported algorithms are between those.
                 pattern = '%s [a-f0-9]{32,128}' % self.algorithm