webadmin: Added external ID to general sub tab

Added external ID for external networks to the general sub tab.

Change-Id: I434a25f0f3c33f40d88d0054921abca2f6cf415a
Bug-Url: https://bugzilla.redhat.com/1067469
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>