tunneldigger-broker: add option to isolate bridge ports

tunneldigger-broker: add option to isolate bridge ports

Add new option to a config bridge section to indicate
if a bridge port added to the bridge should be isolated
or not.  The default is 0 (no isolation).

example

config bridge
     option interface 'br-mybridge1446'
     option mtu '1446'
     option isolate '1' # default '0'

Signed-off-by: Perry Melange <isprotejesvalkata@gmail.com>