Http Server: Add file mapping

There are few scenarios where the httpserver should return files from
the disk in reply to a request.

The json api that are located under /usg/mgmt/api are mapped to
/api/{path}

so a call to
http://192.168.122.89:8000/api/api-docs.json
should return the api doc

The Swagge libray, that is located under /usr/mgmt/swagger-ui is mapped
to /ui so a call to
http://192.168.122.89:8000/ui/index.html
should open the Swagger GUI

The swagger index.html is mapped to /ui so a call to
http://192.168.122.89:8000/ui would also open the Swagger gui

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Http Server: Add file mapping to main

main should call the init method to initilize file mapping

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>