Actually set the caching key prefix

This was accidentally missing in a rebase of
https://github.com/crowbar/barclamp-nova_dashboard/pull/121

(cherry picked from commit cd434536e1bab217bdcc95cf78e789de7580b472)
Syntax error

(cherry picked from commit 655cb21daf343066bdaccabaa2f000473a691aab)
Merge pull request #147 from dirkmueller/release/roxy/master

Actually set the caching key prefix