Fix zero_pad issue in overlay_global

[mikewiebe]

The anycast_gateway_mac property gets zero_padded when it nvgens. Fixes the getter to return a zero-padded value.

Node under test:
  - name  - dt-n9k5-1
  - type  - N9K-C9396PX
  - image - bootflash:///nxos.7.0.3.I3.1.bin

0001.0001.0001
0055.0a10.ffff
1223.3445.5668
TestOverlayGlobal#test_anycast_gateway_mac = 6.54 s = .
TestOverlayGlobal#test_dup_host_ip_addr_detection = 4.11 s = .
TestOverlayGlobal#test_dup_host_mac_detection = 3.94 s = .

Finished in 14.593847s, 0.2056 runs/s, 1.5075 assertions/s.

[mikewiebe]

@chrisvanheuveln did an over the shoulder review.