Savanna: use heat infra engine if available

In Savanna we have two provisioning engines:

* "savanna" that directly work with nova/neutron/glance/cinder/etc and
   we'd like to deprecate it early in Juno release cycle, but it's still
   useful due to the fact that it could work ok w/o Heat;
* "heat" engine uses Heat for provisioning of cluster resources, it's
   currently under active development and we're ready to make it default
   for OpenStack installations with enabled Heat.

Change-Id: I937337b3921e9e51768a118fb4b6bd95962622bd
diff --git a/lib/savanna b/lib/savanna
index 954f0e7..9feff23 100644
--- a/lib/savanna
+++ b/lib/savanna
@@ -135,6 +135,12 @@
         iniset $SAVANNA_CONF_FILE DEFAULT use_floating_ips true
     fi
 
+    if is_service_enabled heat; then
+        iniset $SAVANNA_CONF_FILE DEFAULT infrastructure_engine heat
+    else
+        iniset $SAVANNA_CONF_FILE DEFAULT infrastructure_engine savanna
+    fi
+
     iniset $SAVANNA_CONF_FILE DEFAULT use_syslog $SYSLOG
 
     recreate_database savanna utf8