[VNET] VNET networks are advertised through BGP after warm-reboot or config reload

[veber-alex]

Description

When VNET configuration is saved to config_db and "config reload" or "warm-reboot" are done, FRR configuration is updated to advertise VNET network.

The networks are added to FRR configuration under " address-family ipv4 unicast" as another network to advertise.

Steps to reproduce the issue:

1. Configure VNET and attach a RIF.
2. Save the configuration to config_db.json
3. Perform a "config reload" or "warm-reboot"

Describe the results you received:
VNET RIF network is advertised in BGP through FRR.

Describe the results you expected:
VNET networks should not be advertised in BGP.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

SONiC Software Version: SONiC.201911.0-c15674ee
Distribution: Debian 9.12
Kernel: 4.9.0-9-2-amd64
Build commit: c15674ee
Build date: Mon Feb 17 09:31:47 UTC 2020
Built by: stepanb@r-build-sonic03

Platform: x86_64-mlnx_msn2700-r0
HwSKU: Mellanox-SN2700-D48C8
ASIC: mellanox
Serial Number: MT1811X06319
Uptime: 09:12:37 up 6 days, 20:06,  2 users,  load average: 3.14, 3.26, 3.54

[prsunny]

Fix as part of - #4260

[xinliu-seattle]

Fix checked in, please close this after validation.