Raise baremetal.unprovision_timeout default to 300 sec

In Kilo, Ironic introduced Node cleaning - an optional (but default
enabled) step which is performed after deletion and before returning a
node to the available pool.

Devstack change https://review.openstack.org/#/c/168175/ overrides the
default to 300 already. This patch is just copying that default into
Tempest's config.

Co-authored-by: SHIGEMATSU Mitsuhiro <shigematsu.mitsuhiro@lab.ntt.co.jp>
Change-Id: Icd5fe47bbaba189ef446f976f01309a7e3560a69
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index b3aca42..56172f8 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -161,8 +161,9 @@
 # Timeout for Ironic power transitions. (integer value)
 #power_timeout = 60
 
-# Timeout for unprovisioning an Ironic node. (integer value)
-#unprovision_timeout = 60
+# Timeout for unprovisioning an Ironic node. Takes longer since Kilo
+# as Ironic performs an extra step in Node cleaning. (integer value)
+#unprovision_timeout = 300
 
 
 [boto]