Remove usage of deprecated config 'resources_prefix'

Config option 'resources_prefix' was deprecated and marked for removal.
- https://github.com/openstack/tempest/blob/6dd6fc66d8a04bdba8063f34244e7db2f70a7251/tempest/config.py#L1081
Tempest is going to remove this config options soon.

This commit removes its usage and hardcode the resource prefix to 'tempest'

Change-Id: Ia65b9c8de0bb4abef040ddf0f2f7dd2f3779dcd7
diff --git a/manila_tempest_tests/tests/api/base.py b/manila_tempest_tests/tests/api/base.py
index e81c5d4..b34c92a 100644
--- a/manila_tempest_tests/tests/api/base.py
+++ b/manila_tempest_tests/tests/api/base.py
@@ -177,7 +177,7 @@
             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,
-            resource_prefix=CONF.resources_prefix,
+            resource_prefix='tempest',
             identity_admin_endpoint_type=identity_admin_endpoint_type,
             identity_uri=identity_uri)