Factor up config dependent credential classes

Separate helpers methods to build credentials, credential providers and
client managers that depend on configuration. They are all moved to
the common.credentials module to achieve two objectives:
- prepare credential provider module to move to tempest-lib
- avoid circular dependencies

There are still dependencies to CONF in the credential providers,
removing all of them would make the patch too large; but there is
with this patch a single place where all relevant CONF parameters can
be looked up and passed into the credential providers.

Partially-implements: bp tempest-library
Change-Id: I351f86f0570070105bbf24f2d904e917feea6e57
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index f228040..2cc4b83 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -127,16 +127,16 @@
 unexpected failures in some tests.
 
 
-Non-locking test accounts (aka credentials config options)
-""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+Legacy test accounts (aka credentials config options)
+"""""""""""""""""""""""""""""""""""""""""""""""""""""
 **Starting in the Liberty release this mechanism was deprecated and will be
 removed in a future release**
 
 When Tempest was refactored to allow for locking test accounts, the original
 non-tenant isolated case was converted to internally work similarly to the
-accounts.yaml file. This mechanism was then called the non-locking test accounts
-provider. To use the non-locking test accounts provider you can specify the sets
-of credentials in the configuration file like detailed above with following 9
+accounts.yaml file. This mechanism was then called the legacy test accounts
+provider. To use the legacy test accounts provider you can specify the sets of
+credentials in the configuration file like detailed above with following 9
 options in the identity section:
 
  #. username