The default location for the stir_shaken cache is
/var/lib/asterisk/keys/stir_shaken/cache but we were only creating
/var/lib/asterisk/keys/stir_shaken on istall.  We now create
the cache sub-directory.

Resolves: #634
(cherry picked from commit 215424aa76e1755882d5e9ae18197eaee46ede96)