Tighten cpuset detection code.
Tighten cpuset detection code.

Previous version was sensitive to the hierarchy index that the
cpuset subsystem was mounted as.  In the process of fixing that,
make use of the kernel provided cpuset attribute if available.

BUG=chromium-os:26913
TEST=Find a system where cpuset is in hierarchy!=1 to validate the
     fix (validation requires inspecting cros/cbuildbot/* grouping
     to ensure it nests).

Change-Id: I970bc72f40eaaec28e7ec3e888d58e14fec4f868
Reviewed-on: https://gerrit.chromium.org/gerrit/17299
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

zhang0137/chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite