Update README.md for running cinder tests in tempest

Two changes to README.md are made:
1. Fixed command line for calling cinder-tempest-plugin tests using plugin name
2. Removed usage of deprecated tox command [all-plugin -> all].
https://github.com/openstack/tempest/blob/master/tox.ini#L55

Change-Id: Ic2c88acfba606592dd12677cc70649ff545c1222
diff --git a/README.rst b/README.rst
index f42d0eb..79151cd 100644
--- a/README.rst
+++ b/README.rst
@@ -10,10 +10,10 @@
 To run all tests from this plugin, install cinder into your environment. Then
 from the tempest directory run::
 
-    $ tox -e all-plugin -- volume
+    $ tox -e all -- cinder_tempest_plugin
 
 
-It is expected that Cinder third party CI's use the all-plugin tox environment
+It is expected that Cinder third party CI's use the `all` tox environment
 above for all test runs. Developers can also use this locally to perform more
 extensive testing.