Fix race condition in delete_port method. Fix update_port method

Port can be gone between
  l3plugin.prevent_l3_port_deletion(context, id)
and port query. Need to handle it properly.
ALso need to handle non-existing port in update_port properly.

Closes-Bug: #1266537

Change-Id: Ic203f21db277f83c604a8757a8c421b3fb9ae709
Merge "Fix race condition in delete_port method. Fix update_port method"