qa: resolve regression picked up in test qa/042
qa: resolve regression picked up in test qa/042

Recent tweaks to exported containers.cgroup metric value
(for Vector) meant the PDU REQ had an unexpected prefix,
and pmdaproc cgroup scanning no longer worked as before.

wuliming/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp