[sonic_package_manager] flush once finished saving docker image into temporary file (#1638)

Since installation happens inside context manager "with tempfile.NamedTemporaryFile()"
it may be that depending on file size/number of writes the docker image isn't fully written
to the disk. In practice a small test docker image was succesfully migrated but when trying with
larger onces it started to file. This change fixes the issue by adding a flush.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

Author: stepanblyschak

sonic_package_manager/manager.py

@@ -638,6 +638,7 @@ def migrate_package(old_package_entry,
                 with tempfile.NamedTemporaryFile('wb') as file:
                     for chunk in image.save(named=True):
                         file.write(chunk)
+                    file.flush()
 
                     self.install(tarball=file.name)
             else: