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
1 file changed