Support bridge in cloud

Nat take advantage of that and the code is more simple.

Fix #761

gns3server/utils/interfaces.py

@@ -16,6 +16,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
+import os
 import sys
 import aiohttp
 import socket
@@ -163,6 +164,13 @@ def is_interface_up(interface):
         return True
 
 
+def is_interface_bridge(interface):
+    """
+    :returns: True if interface is a bridge
+    """
+    return os.path.exists(os.path.join("/sys/class/net/", interface, "bridge"))
+
+
 def _check_windows_service(service_name):
 
     import pywintypes