Allow to use a relative path to dlls location

Problem/Motivation

It is currently not possible to use a relative path started with "/" as a DLLs location value because the "/" is removed on form submit and when retrieving the variable value.

This is a blocker to store DLLs location locally as the url is not relative and become incorrect in certains conditions / paths.

Steps to reproduce

1. Store dlls locally (for example in a custom module)
2. Set the relative path to dlls (for example : /modules/custom/yourmodule/dll/)
3. Disable javascript aggregation
4. Edit a node with a ckeditor field
5. See that an error occured : `GET http://yoursite/node/452/modules/custom/yourmodule/dll/cloud-services/cl... net::ERR_ABORTED 404 (Not Found)`

Proposed resolution

Do not remove the initial "/" of the variable so it will be possible to use a relative local path.