Convert compute api tests to global CONF object

This commit takes all the uses of config in the compute api tests and
converts them to use the global CONF object.

Partially implements bp config-cleanup

Change-Id: Iac92d6ac88200222392a131b4c95411594c509ba
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index 8f49aec..e0181b9 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -19,9 +19,12 @@
 from tempest.api.compute import base
 from tempest import clients
 from tempest.common.utils import data_utils
+from tempest import config
 from tempest import exceptions
 from tempest import test
 
+CONF = config.CONF
+
 
 class ServersNegativeTestJSON(base.BaseV2ComputeTest):
     _interface = 'json'
@@ -408,7 +411,7 @@
         self.assertEqual(202, resp.status)
         self.addCleanup(self.client.unshelve_server, self.server_id)
 
-        offload_time = self.config.compute.shelved_offload_time
+        offload_time = CONF.compute.shelved_offload_time
         if offload_time >= 0:
             self.client.wait_for_server_status(self.server_id,
                                                'SHELVED_OFFLOADED',