Fixing lost vlan ids on interfaces

Sometimes a vm gets its tap interface unset and reset too fast to be caught in
an agent loop, and its vlan tag was not reset.

We now detect if an interface loses its vlan tag, and if it happens the
interface will be reconfigured.

Since the TAG ID is only available via the "Port" table (in the 'tag' column),
we couldn't reuse the get_vif_port_set() method's run_vsctl call which queries
the "Interface" table, and needed a specific run_vsct call to the "Port" table
in the new get_port_tag_dict() method.

Change-Id: I7f59e2c1e757c28dae35c44ebfad9d764ae1d3c5
Closes-Bug: 1240849
Merge "Fixing lost vlan ids on interfaces"