tempest: start using get_configured_admin_credentials

Tempest removed get_configured_credentials in [1].
This patch aims to update the way we get admin credentials in Tempest
client code.

[1] https://github.com/openstack/tempest/commit/bc0a7a6d8022c7240f5f1f80c0c93b43e9037ad4

Change-Id: Ic97c011f8beec8df4253949285794be21b10f7cf
diff --git a/ironic_tempest_plugin/clients.py b/ironic_tempest_plugin/clients.py
index 2cb7c73..7816a2f 100644
--- a/ironic_tempest_plugin/clients.py
+++ b/ironic_tempest_plugin/clients.py
@@ -22,7 +22,7 @@
 
 CONF = config.CONF
 
-ADMIN_CREDS = common_creds.get_configured_credentials('identity_admin')
+ADMIN_CREDS = common_creds.get_configured_admin_credentials()
 
 
 class Manager(clients.Manager):