Use dsconf to add lines to devstack localrc

In some situations there are additional sections in the devstack
localrc file and they may be at the end of the file. This caused
errors as the desired changes would not be placed in the
'[[local|localrc]]' section of the file.

Using 'dsconf setlc_raw' solves that issue.

Change-Id: If4bc197d685e20fadbc5ffbd2151cae9d8129135

inc/plugin

@@ -166,8 +166,8 @@ function devstack_localrc {
     local path=$(localrc_path $where)
     shift
     echo "Adding settings to $where at $path"
-    echo "# added by $settings_file" >> $path
-    echo "$@" >> $path
+    dsconf setlc_raw $path "# added by $settings_file"
+    dsconf setlc_raw $path "$@"
 }
 
 function fetch_grenade_plugins {