Make credentials_factory a bit nicer

The helpers to get parameters for the credential providers were set
up some time ago but not updated anymore, so we have now parameters
passed in get_credentials_provider directly, and some obtained
through the helpers.

Fixing this and adding some docstring. This work is in preparation
to make the module stable for plugins, so that it can be used
by test.py.

Change-Id: Ifee212277e8c1282a54b5443a51531ff731be879
diff --git a/tempest/cmd/account_generator.py b/tempest/cmd/account_generator.py
index 172d9e1..a76123c 100755
--- a/tempest/cmd/account_generator.py
+++ b/tempest/cmd/account_generator.py
@@ -141,18 +141,10 @@
     admin_creds = credentials_factory.get_credentials(
         fill_in=False, identity_version=identity_version, **admin_creds_dict)
     return dynamic_creds.DynamicCredentialProvider(
-        identity_version=identity_version,
         name=opts.tag,
         network_resources=network_resources,
-        neutron_available=CONF.service_available.neutron,
-        create_networks=CONF.auth.create_isolated_networks,
-        identity_admin_role=CONF.identity.admin_role,
-        identity_admin_domain_scope=CONF.identity.admin_domain_scope,
-        project_network_cidr=CONF.network.project_network_cidr,
-        project_network_mask_bits=CONF.network.project_network_mask_bits,
-        public_network_id=CONF.network.public_network_id,
-        admin_creds=admin_creds,
-        **credentials_factory.get_dynamic_provider_params())
+        **credentials_factory.get_dynamic_provider_params(
+            identity_version, admin_creds=admin_creds))
 
 
 def generate_resources(cred_provider, admin):