ceph.spec.in:  Move four scripts from sbin to usr/bin

The ceph-create-keys, ceph-disk, ceph-disk-activate, and
ceph-disk-prepare scripts are built in sbin, but debian installs
them into usr/bin, and several utilities look for them there.
This commit changes the RPM to install them in /usr/bin. (Bug #3921)

Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>