Make test_drivers.py use driver name from conf

test_list_drivers and test_show_driver in test_drivers.py use hard_coded
driver name.
This patch make these tests use driver name which is written in config file.

Closes-Bug: #1317717

Change-Id: Idad4c496a42c9253548fdf9020ffb8013fef951f
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 2ed2582..858fce9 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -109,6 +109,9 @@
 # value)
 #driver_enabled=false
 
+# Driver name which Ironic uses (string value)
+#driver=fake
+
 # The endpoint type to use for the baremetal provisioning
 # service (string value)
 #endpoint_type=publicURL