Use a host specific memcache key for django_compressor

Otherwise all but one node in the HA cluster will miss
the static assets, causing ugliness and 404 messages in
the browser.

Fixes
https://bugzilla.novell.com/show_bug.cgi?id=894070

(cherry picked from commit f8853688e7091a18236852d50f2008f07b4db6aa)
Merge pull request #187 from dirkmueller/release/roxy/master

Use a host specific memcache key for django_compressor