rest_api: expose set/get hostname api

This patch is for mgmt.

This allows to get or set the hostname via the rest api.
Example:
Get the hostname:
curl http://localhost:8000/os/hostname
Set the hostname to 'osv_rocks'
curl -X POST http://localhost:8000/os/hostname?name=osv_rocks

Fixes #60

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>