Asset Fetcher: remove hashfile attribute

In another part of the purge of attributes, let's remove hashfile.
This attribute is yet another case of an attribute that is not
intended for end users, and basically serves as a way to pass
information across methods.

Because the hash file name is so easily computable, instead of turning
it into a private, let's just evaluate the hash file name when needed.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/asset.py

@@ -95,6 +95,13 @@ class Asset(object):
                 return cache_dir
         raise EnvironmentError("Can't find a writable cache directory.")
 
+    @staticmethod
+    def _get_hash_file(asset_file):
+        """
+        Returns the file name that contains the hash for a given asset file
+        """
+        return '%s-CHECKSUM' % asset_file
+
     def fetch(self):
         """
         Fetches the asset. First tries to find the asset on the provided
@@ -116,7 +123,6 @@ class Asset(object):
         for cache_dir in self.cache_dirs:
             cache_dir = os.path.expanduser(cache_dir)
             asset_file = os.path.join(cache_dir, basename)
-            self.hashfile = '%s-CHECKSUM' % asset_file
 
             # To use a cached file, it must:
             # - Exists.
@@ -137,7 +143,6 @@ class Asset(object):
         # writable cache directory will be used.
         cache_dir = self._get_writable_cache_dir()
         asset_file = os.path.join(cache_dir, basename)
-        self.hashfile = '%s-CHECKSUM' % asset_file
 
         # Now we have a writable cache_dir. Let's get the asset.
         # Adding the user defined locations to the urls list:
@@ -180,15 +185,16 @@ class Asset(object):
 
     def _compute_hash(self, asset_file):
         result = crypto.hash_file(asset_file, algorithm=self.algorithm)
-        with open(self.hashfile, 'w') as f:
+        with open(self._get_hash_file(asset_file), 'w') as f:
             f.write('%s %s\n' % (self.algorithm, result))
 
     def _get_hash_from_file(self, asset_file):
         discovered = None
-        if not os.path.isfile(self.hashfile):
+        hash_file = self._get_hash_file(asset_file)
+        if not os.path.isfile(hash_file):
             self._compute_hash(asset_file)
 
-        with open(self.hashfile, 'r') as hash_file:
+        with open(hash_file, '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.