Add rule for updating network's router:external attribute

Set admin_only rule for update_network:router:external in policy.json

Also, change the default value of router:external from attr.ATTR_NOT_SPECIFIED
to False, because each time we try to get or update a network the dict with
its attributes is extended by _extend_network_dict_l3 function which adds
router:external=False to the dict if this attribute is not specified.
Thus, if the default value is not specified, router:external is considered
to be updated in any case and the policy rule is applied.

Change-Id: I899d98c7d8c9d9863ac5d8f992b6a2d507ec4482
Closes-Bug: 1338880
ML2 mechanism driver for SR-IOV capable NIC based switching, Part 2

This set of changes introduces SRIOV NIC Agent to run with
ML2 mechanism driver for SR-IOV capable NIC based switching.
This is the second part of a 2 part commit.

The review is submitted in two parts:
- Part 1
    The Mechanism Driver to support port binding for SR-IOV virtual
	functions of SRIOV capable switching NICs.

- Part2 (this part)
    The SRIOV NIC Based L2 Agent.
	Use  configurable list of mappings physical_networks to PF
        interfaces and configurable list of mappings PF interfaces
        to list of excluded VFs to get list of Virtual Functions that agent should manage.
	Current implementation supports admin state updates.

Co-authored-by: Samer Deeb <samerd@mellanox.com>
Partially implements: blueprint ml2-sriov-nic-switch

Change-Id: I533ccee067935326d5837f90ba321a962e8dc2a6
Merge "ML2 mechanism driver for SR-IOV capable NIC based switching, Part 2"
Merge "Add rule for updating network's router:external attribute"