Switch to using subunit-trace from tempest-lib

This commit removes the tempest copy of subunit-trace and uses the
entry-point from tempest-lib instead.

Partially implements bp tempest-library

Change-Id: Ic1aeddc2d0b217c41b6b9914ac3f269b80c4ec8e
Merge "Switch to using subunit-trace from tempest-lib"