Minimise memory footprint during upload.

There is no need to read whole file into memory on library level. So calculate
sum in chunks and give request method file descriptor.

slapos/libnetworkcache.py

@@ -68,14 +68,21 @@ class NetworkcacheClient(object):
       msg = 'The parameter "urlmd5" is required once you set directory_key.'
       raise ValueError(msg)
 
-    file_content = file_descriptor.read()
-    sha512sum = hashlib.sha512(file_content).hexdigest()
-    path = os.path.join(self.shacache_path, sha512sum)
-
+    sha512sum = hashlib.sha512()
+    # do not trust, go to beginning of opened file
+    file_descriptor.seek(0)
+    while True:
+      d = file_descriptor.read(sha512sum.block_size)
+      if not d:
+        break
+      sha512sum.update(d)
+    path = os.path.join(self.shacache_path, sha512sum.hexdigest())
+
+    file_descriptor.seek(0)
     shacache_connection = httplib.HTTPConnection(self.shacache_host,
       self.shacache_port)
     try:
-      shacache_connection.request('PUT', path, file_content,
+      shacache_connection.request('PUT', path, file_descriptor,
                                                  self.shacache_header_dict)
       result = shacache_connection.getresponse()
       data = result.read()