[ut] replace .testr.conf with .stestr.conf

In Chaneg[1], os_testr switches the otestr command to use stestr
under the covers. So that we get warning:

ostestr.py:120:
 UserWarning: No .stestr.conf file found in the CWD.
 Please create one to to replace the .testr.conf. You can find a
 script to do this in the stestr git repository.

So remove the .testr.conf and add .stestr.conf which was generated
using a utility script as documented here[2]:

[1] Id7cb2a39a8308f1413608dcf19273a1d7f33592e
[2] https://media.readthedocs.org/pdf/stestr/stable/stestr.pdf

Change-Id: Ibc39d9c3fe33be91be158e7f6cd574736ed89440

.gitignore

@@ -5,6 +5,7 @@
 .coverage
 .testrepository
 .tox
+.stestr
 .venv
 .idea
 AUTHORS

.stestr.conf

@@ -0,0 +1,3 @@
+[DEFAULT]
+test_path=./sahara/tests
+top_dir=./