LinuxBridge: update status according to admin_state_up

Fixes bug 1099065

In addition to this the agent will only treat ports that exist on the agent.

Change-Id: I927649a45a860421ef0d825015516000475ad08d
LinuxBridge: set port status as 'DOWN' on creation

When a port is created the initial status will be 'DOWN'. The agent
in turn will update the plugin of the actual port status.

Change-Id: I66b51501304f7cb3f0157a4adaec4b82c62b30a3