cros_sdk: contain children in a cgroup.
cros_sdk: contain children in a cgroup.

This is done to ensure that no children escape cros_sdk.

BUG=chromium-os:26913
TEST=cros_sdk

Change-Id: I3580901d592af366776d7c64c5949dbd6039ab4f
Reviewed-on: https://gerrit.chromium.org/gerrit/17077
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

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