ostestr is no longer installed by default by infra and will be totally
removed longer term; stestr  is its replacement.
This is the reason out gate is failing with the following today:
nsx/tools/ostestr_compat_shim.sh: ostestr: not found
This patch moves us to stestr by:
- Updating tox.ini to use stestr rather than call ostestr_compat_shim.sh
and in turn ostestr.
- Removing ostestr_compat_shim.sh as its no longer used.
For more details on stestr see .
Also note that this is a quick migration to stestr to get the gate
working; additional tweaks may come up.