[V3] Rename get_credential to show_credential

[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to credentials client.

Partially implements blueprint consistent-service-method-names

Change-Id: I07c4403e42d6f2c7617f455a8a4f292ff10e7059
diff --git a/tempest/services/identity/v3/json/credentials_client.py b/tempest/services/identity/v3/json/credentials_client.py
index 7d4cf9a..753e960 100644
--- a/tempest/services/identity/v3/json/credentials_client.py
+++ b/tempest/services/identity/v3/json/credentials_client.py
@@ -51,7 +51,7 @@
         body['credential']['blob'] = json.loads(body['credential']['blob'])
         return service_client.ResponseBody(resp, body)
 
-    def get_credential(self, credential_id):
+    def show_credential(self, credential_id):
         """To GET Details of a credential."""
         resp, body = self.get('credentials/%s' % credential_id)
         self.expected_success(200, resp.status)