Compress response's content according to client's accepted encoding

Currently Glance ignores the Accept-Encoding header and returns
responses as they are regardless the client accepts gzip or other type
of compression.

This patch adds this capability to glance (by using a middleware)
supporting just gzip for now.

Important note:
    - The patch uses a lazy compression for Content-Type
    application/octet-stream but in order to do that, the
    content-length has to be unset which means that when an image is
    downloaded the content-length will be unknown to the client.

Fixes bug: 1150380

Change-Id: Ieb65837d4e3fe310f97d9666882ecc572b14956a