Move ostree-* executables to /usr/lib/ostree

Why not to use libexecdir?

Because this directory does not exist on some distros or paths
between distros varies. There are several reasons why a well
known path is prefered, for example when generating a kernel
command line:

init=${ostree}/usr/lib/ostree-boot/ostree-prepare-root

In addition this saves us some typing in a console when wanting
to access the "ostree" cmd line.

Closes: #449
Approved by: cgwalters

src/boot/ostree-prepare-root.service

@@ -27,7 +27,7 @@ Before=plymouth-switch-root.service
 
 [Service]
 Type=oneshot
-ExecStart=/usr/sbin/ostree-prepare-root /sysroot
+ExecStart=/usr/lib/ostree/ostree-prepare-root /sysroot
 StandardInput=null
 StandardOutput=syslog
 StandardError=syslog+console