Fix file location for nova compute rbd secret

The rbd secret location was mixed between /etc/nova
and /etc/ceph. This patch changes the secret to
be in /etc/nova for both the file and exec
resources so that virsh secrets are set correctly

Change-Id: I50f0324d911f20d5a072c769edcf289ae59f817a
(cherry picked from commit aad7a0a7cbb2770d4d1f668c1b8fe871cd7d7205)