usertools: fix CPU layout for more than 2 threads

usertools: fix CPU layout for more than 2 threads

Current usertools/cpu_layout.py is broken to handle multithreads
of count more than 2 as in IBM powerpc P8 servers.
Below patch addressed this issue.

Also, added minor exception catch on failing to open unavailable
sys file in case of multithread=off configuration in server.

Patch has been verified not to break existing topology configurations
and also not changing anything in current output.

Signed-off-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>
Reviewed-by: Andriy Berestovskyy <andriy.berestovskyy@caviumnetworks.com>