Add [baremetal] available_nodes configuration option

This patch adds the available_nodes configuration option so that when a
test requires more than one node it can be skipped if it is known that
there are not enough nodes in the environment to test that test.

Change-Id: I3a7809b2c4b06f429900bd40f4576b1bf8a50891
diff --git a/ironic_tempest_plugin/config.py b/ironic_tempest_plugin/config.py
index f9fefc4..887e6ad 100644
--- a/ironic_tempest_plugin/config.py
+++ b/ironic_tempest_plugin/config.py
@@ -120,6 +120,9 @@
                min=0,
                help="Ironic adjusted disk size to use in the standalone tests "
                     "as instance_info/root_gb value."),
+    cfg.IntOpt('available_nodes', min=0, default=None,
+               help="The number of baremetal hosts available to use for "
+                    "the tests.")
 ]
 
 BaremetalFeaturesGroup = [