[vlanmgrd]: Fixing an issue causing mismatch between MAC and link-local IPv6 addresses of VLAN and Bridge interfaces

[mramezani95]

What I did

1. Added code to bring down the Bridge interface before changing the MAC address of a VLAN interface and the Bridge, and then starting up the Bridge after the MAC is changed. This automatically brings down and starts up all VLAN interfaces added to the Bridge.
2. Added code to bring down and then immediately start up the Bridge after the dummy interface is started up. This is useful to ensure that MAC and link-local IPv6 addresses of the Bridge are consistent in case no VLANs are added to the Bridge later.
3. Added a VS test to verify these behaviors.

Why I did it
After PR #3370, the Bridge becomes operationally UP after the dummy interface is started up. As a result, all VLAN interfaces created under the Bridge are immediately operationally UP after creation. This can cause an issue later if their MAC address is changed since the kernel does not update the link-local IPv6 address of an interface if it is operationally UP. This behavior caused the IPv6 version of the following test cases in sonic-mgmt to fail for the dualtor topology, which were temporarily skipped:

arp/test_arp_dualtor.py::test_arp_update_for_failed_standby_neighbor
arp/test_arp_dualtor.py::test_standby_unsolicited_neigh_learning
arp/test_arp_extended.py::test_proxy_arp

How I verified it

1. Verified that the above sonic-mgmt tests can be passed after this change.
2. Verified that the link-local IPv6 addresses are updated correctly after the Bridge is brought down and started up again.

[mssonicbld]

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[mramezani95]

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[mramezani95]

/azpw run

[mssonicbld]

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[mssonicbld]

/azp run

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