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
Regression caused by commit b56c2c998

Fixes bug 1109001

Change-Id: I5f0304cca380813e552cd229635a9aacee42126a