Rudder

We want to provide a new field type for directives that allows (but not force) to use a graphical explorer to select the path of a file from shared file.

In that first version, there is no specific rights associated with the file explorer: if you are able to modify directive, you can use the file explorer.

Bonus point if the file explorer allows to see the content of the file (but not to edit it) and if it is configurable (i.e, if we somehow have an option to enable/disable it globally).

So we need:

- a new type of VType in /rudder-core/src/main/scala/com/normation/cfclerk/domain/Constraint.scala and its parsing in /rudder-core/src/main/scala/com/normation/cfclerk/xmlparsers/VariableSpecParser.scala => parseConstraint
- a new field type in /rudder-web/src/main/scala/com/normation/rudder/web/model/DirectiveFieldEditors.scala
- a new mapping between the two in /rudder-web/src/main/scala/bootstrap/liftweb/AppConfig.scala => FieldFactoryImpl (I'm seeing that there is already an UploadedFileVType that can be used as a model)

The first step is to find and test a file directory browser. The only one I know for angular is that one: https://github.com/joni2back/angular-filemanager/blob/master/README.md