OVS: update status according to admin_state_up

Fixes bug 1099099

In addition to this the code does the following:
1. validates if the port exists on the OVS when a port update event takes
   place
2. when a port is created the initial state is set as 'DOWN'. this is
   later updated by the agent when it is detected as up

Change-Id: I917d4b5aa8df7395fcaa841d77083546478739d2
Merge "OVS: update status according to admin_state_up"