refactor v3 cred tests to work with pre-prov

This commit aims to use existing credentials to execute tests:
  * test_credentials_list_delete
  * test_credentials_create_get_update_delete
so that users with a cloud using the pre-provisioned credential
provider can execute these tests.

Change-Id: Ida753ad4f550eeeec47b6f34b73c936b0eab3d71
Partial-Bug: #1714277
diff --git a/tempest/api/identity/admin/v3/test_credentials.py b/tempest/api/identity/admin/v3/test_credentials.py
index ba19ff7..23fe788 100644
--- a/tempest/api/identity/admin/v3/test_credentials.py
+++ b/tempest/api/identity/admin/v3/test_credentials.py
@@ -20,6 +20,10 @@
 
 
 class CredentialsTestJSON(base.BaseIdentityV3AdminTest):
+    # NOTE: force_tenant_isolation is true in the base class by default but
+    # overridden to false here to allow test execution for clouds using the
+    # pre-provisioned credentials provider.
+    force_tenant_isolation = False
 
     @classmethod
     def resource_setup(cls):
@@ -27,10 +31,6 @@
         cls.projects = list()
         cls.creds_list = [['project_id', 'user_id', 'id'],
                           ['access', 'secret']]
-        u_name = data_utils.rand_name('user')
-        u_desc = '%s description' % u_name
-        u_email = '%s@testmail.tm' % u_name
-        u_password = data_utils.rand_password()
         for _ in range(2):
             project = cls.projects_client.create_project(
                 data_utils.rand_name('project'),
@@ -38,12 +38,8 @@
             cls.addClassResourceCleanup(
                 cls.projects_client.delete_project, project['id'])
             cls.projects.append(project['id'])
-
-        cls.user_body = cls.users_client.create_user(
-            name=u_name, description=u_desc, password=u_password,
-            email=u_email, project_id=cls.projects[0])['user']
-        cls.addClassResourceCleanup(
-            cls.users_client.delete_user, cls.user_body['id'])
+        cls.user_body = cls.users_client.show_user(
+            cls.os_primary.credentials.user_id)['user']
 
     def _delete_credential(self, cred_id):
         self.creds_client.delete_credential(cred_id)