Update the README file to use tox -e all

This patch updates the README file to have instructions for running the tempest tests using the "all" environment instead of "all-plugins". The "all-plugins" environment has been deprecated for years and uses site-packages, which may cause unintended test scenarios.
It also updates the example "run a single test" to be a non-admin API test.

Change-Id: I8645fb7577d52cd35e3d61d44ca9028ea3afe00f
diff --git a/README.rst b/README.rst
index e1a2d04..902b26d 100644
--- a/README.rst
+++ b/README.rst
@@ -23,15 +23,25 @@
 Running the tests
 -----------------
 
+From the tempest directory, setup the tempest virtual environment for the
+designate tempest plugin::
+
+    $ tox -e venv-tempest -- pip3 install -e <path to designate-tempest-plugin>
+
+For example, when using a typical devstack setup::
+
+    $ cd /opt/stack/tempest
+    $ tox -e venv-tempest -- pip3 install -e /opt/stack/designate-tempest-plugin
+
 To run all tests from this plugin, install designate into your environment
 and from the tempest repo, run::
 
-    $ tox -e all-plugin -- designate
+    $ tox -e all -- designate
 
 To run a single test case, run with the test case name, for example::
 
-    $ tox -e all-plugin -- designate_tempest_plugin.tests.api.v2.test_zones.ZonesAdminTest.test_get_other_tenant_zone
+    $ tox -e all -- designate_tempest_plugin.tests.api.v2.test_zones.ZonesTest.test_create_zones
 
 To run all tempest tests including this plugin, run::
 
-    $ tox -e all-plugin
+    $ tox -e all