feat(CpuUtils): Allow suites to forbid siblings

feat(CpuUtils): Allow suites to forbid siblings

Some perf suites use a single unidirectional flow of traffic.
On SMT-enabled machines, 1C tests create 2 workers by default.
But if there is only one stream/flow,
only one worker does useful work.
The other worker on the sibling logical core
does busy-wait, which provably lowers the peformance.

Some suites already set smt_used (as a suite variable)
to False for these purposes.
This change edits a lower-level L1 keyword to honor this value,
and adds the suite variable to single stream (and client) hoststack
suites, where the detrimental effect of busy siblings was observed.

Change-Id: I203f1a700d8e6cd84640b9c0b40487cb832908aa
Signed-off-by: Vratko Polak <vrpolak@cisco.com>