fix bug #253 "NetworkManager runs DHCP..." for Ubuntu (but not Fedora/CentOS)

use "vethA.B.SS" name for veth devices (A = node number, B = index, SS = short session ID)
use "tapA.B.SS" name for tap devices; use "b.A.SS" name for bridge devices
added a SESSION_SHORT environment variable to the default CORE environment
(Boeing r1867)

daemon/core/netns/vnet.py

@@ -211,7 +211,8 @@ class LxBrNet(PyCoreNet):
         if policy is not None:
             self.policy = policy
         self.name = name
-        self.brname = "b.%s.%s" % (str(self.objid), self.session.sessionid)
+        sessionid = self.session.shortsessionid()
+        self.brname = "b.%s.%s" % (str(self.objid), sessionid)
         self.up = False
         if start:
             self.startup()