Use reboot from /sbin

According to FHS, the reboot command should live in /sbin.
Systems may also have a symlink in /usr/bin, but they don't have to.

https://fedorahosted.org/freeipa/ticket/2480

ipa-client/ipa-install/ipa-client-install

@@ -409,7 +409,7 @@ def uninstall(options, env, quiet=False):
         if not options.on_master:
             if user_input("Do you want to reboot the machine?", False):
                 try:
-                    run(["/usr/bin/reboot"])
+                    run(["/sbin/reboot"])
                 except Exception, e:
                     emit_quiet(quiet, "Reboot command failed to exceute. " + str(e))
                     return CLIENT_UNINSTALL_ERROR