Fix remove_disabled_extensions to remove an extension at the last position

remove_disabled_extensions do matching by '$ext_to_remove","'. So it doesn't
match an extension at the last position in extensions_list.
This patch fixes that.

Closes-Bug: #1443254
Change-Id: I194b483de797697ba06b320cf33f1bac67fc0cc7
diff --git a/lib/tempest b/lib/tempest
index 6c34323..ffb59ff 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -91,10 +91,7 @@
     local extensions_list=$1
     shift
     local disabled_exts=$*
-    for ext_to_remove in ${disabled_exts//,/ } ; do
-        extensions_list=${extensions_list/$ext_to_remove","}
-    done
-    echo $extensions_list
+    remove_disabled_services "$extensions_list" "$disabled_exts"
 }
 
 # configure_tempest() - Set config files, create data dirs, etc