Remove non-locking accounts from the config guide

Before Tempest had three different methods of providing authentication,
dynamic credentials, locking test accounts and non-locking test
accounts, currently only the first two are supported. This commit
updates a section on the configuration guide where it still mentions
three methods.

Change-Id: I532aff27a803b9fd34e994153f4183de48d91d00
Closes-Bug: 1600319
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index e4b104f..5386712 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -61,10 +61,9 @@
 Credential Provider Mechanisms
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Tempest currently also has three different internal methods for providing
-authentication to tests: dynamic credentials, locking test accounts, and
-non-locking test accounts. Depending on which one is in use the configuration
-of Tempest is slightly different.
+Tempest currently has two different internal methods for providing authentication
+to tests: dynamic credentials and pre-provisioned credentials.
+Depending on which one is in use the configuration of Tempest is slightly different.
 
 Dynamic Credentials
 """""""""""""""""""