Fix service description

Signed-off-by: John Else <john.else@citrix.com>
CA-121668: Move plugins to init level 23

As part of EA-1254 xcp-rrdd moved to init level 22; ever since then the
plugins have been trying (and failing) to start up before xcp-rrdd.

Signed-off-by: John Else <john.else@citrix.com>