Setup subunit-trace as an entry point

This commit moves subunit-trace into the newly created cmd dir and
adds a pbr entry point to run it as a binary. This will enable using
subunit-trace just by installing tempest-lib.

Change-Id: I3ae379edca3e776e90a18d53393c4c1424d291f7
Merge "Setup subunit-trace as an entry point"