Fix register cluster method in advanced_cluster_mgmt_lib.resource

Change-Id: I02bde6b0d3b34373c37b6c30c9d56a558d2ea22e
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/robot-systest/lib/advanced_cluster_mgmt_lib.resource b/robot-systest/lib/advanced_cluster_mgmt_lib.resource
index 266b94f..84ce1a2 100644
--- a/robot-systest/lib/advanced_cluster_mgmt_lib.resource
+++ b/robot-systest/lib/advanced_cluster_mgmt_lib.resource
@@ -71,9 +71,9 @@
     ...   --creds ${creds}
     ...   --description ${description}
     IF   ${bootstrap_flag} == True
-        Catenate   ${command}   --bootstrap
+        ${command}=   Catenate   ${command}   --bootstrap
     ELSE
-        Catenate   ${command}   --no-bootstrap
+        ${command}=   Catenate   ${command}   --no-bootstrap
     END
     ${rc}   ${stdout}=   Run And Return Rc And Output   ${command}
     Log   ${rc},${stdout}