Flush netcat socket buffer when proxying HTTP connections

HTTP requests from Sahara to Hadoop services can wait indefinitely when
proxied through Neutron namespaces, because buffers are not flushed.
Make sure they are to avoid deadlocks.

Change-Id: Ic1d1b74345be49e405f00264b8c7df3dc06c5b09
Closes-bug: #1384316
(cherry picked from commit 5d0a261217c63903e0a682b673657af3e3bb0e09)
Merge "Flush netcat socket buffer when proxying HTTP connections" into stable/juno