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/tempest/test.py b/tempest/test.py
index b8ba5f4..4be6779 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -32,7 +32,7 @@
 
 from tempest import clients
 from tempest.common import cred_client
-from tempest.common import credentials
+from tempest.common import credentials_factory as credentials
 from tempest.common import fixed_network
 import tempest.common.generator.valid_generator as valid
 import tempest.common.validation_resources as vresources