Return the platform of a compute

Ref https://github.com/GNS3/gns3-gui/issues/1550

gns3server/handlers/api/compute/capabilities_handler.py

@@ -15,6 +15,8 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import sys
+
 from gns3server.web.route import Route
 from gns3server.config import Config
 from gns3server.schemas.capabilities import CAPABILITIES_SCHEMA
@@ -37,5 +39,6 @@ class CapabilitiesHandler:
 
         response.json({
             "version": __version__,
+            "platform": sys.platform,
             "node_types": node_types
         })