Unhardcode instance_build_timeout option

This patch allows to configure instance_build_timeout option that
defines maximum time in seconds that an instance can take to build.

Change-Id: Ia589aa368a87f02df3955697b19bcafe6e6390d7
diff --git a/README.rst b/README.rst
index a290d34..1d3daa9 100644
--- a/README.rst
+++ b/README.rst
@@ -32,6 +32,7 @@
         dhcp_domain: novalocal
         vif_plugging_timeout: 300
         vif_plugging_is_fatal: false
+        instance_build_timeout: 600
         consoleauth:
           token_ttl: 600
         bind: