Http Server: Search a header in the request by its name

The request object holds the headers in a vector. This is because a
header can apear more than once. Sometimes it is nessary to find a
header, where only one header will be found.

The get_header method searches for the first header by its name

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Http Server: Add a file parser

When the http server reads a file from the disk, it is sometimes
required that the files will be parse and string substitution will be
perform.

For example, when reading the json files, it is required to place the
real host name/ip so the Swagger will be able to use those files. The
file parser will look for a parameter in the file (given that it has the
right extension) and will replace it.

Currently it only support {$Host} parameter, additional parameters can
be added when needed

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