networkd: add bridge properties

networkd: add bridge properties

ForwardDelaySec:   forward delay
HelloTimeSec:      hello time
MaxAgeSec:         maximum message age

for more information see
http://www.tldp.org/HOWTO/BRIDGE-STP-HOWTO/set-up-the-bridge.html

In kernel
br_dev_newlink: does not have the this functionality to set while
creation.
br_changelink: after creation we can change the parameters.

we need to first create then set it the parameters.

Introduce new callback post_create .This should
set the properties after the creation.