**- Why I did it**
To introduce dynamic support of BBR functionality into bgpcfgd.
BBR is adding `neighbor PEER_GROUP allowas-in 1' for all BGP peer-groups which points to T0
Now we can add and remove this configuration based on CONFIG_DB entry
**- How I did it**
I introduced a new CONFIG_DB entry:
- table name: "BGP_BBR"
- key value: "all". Currently only "all" is supported, which means that all peer-groups which points to T0s will be updated
- data value: a dictionary: {"status": "status_value"}, where status_value could be either "enabled" or "disabled"
Initially, when bgpcfgd starts, it reads initial BBR status values from the [constants.yml](https://github.com/Azure/sonic-buildimage/pull/5626/files#diff-e6f2fe13a6c276dc2f3b27a5bef79886f9c103194be4fcb28ce57375edf2c23cR34). Then you can control BBR status by changing "BGP_BBR" table in the CONFIG_DB (see examples below).
bgpcfgd knows what peer-groups to change fron [constants.yml](https://github.com/Azure/sonic-buildimage/pull/5626/files#diff-e6f2fe13a6c276dc2f3b27a5bef79886f9c103194be4fcb28ce57375edf2c23cR39). The dictionary contains peer-group names as keys, and a list of address-families as values. So when bgpcfgd got a request to change the BBR state, it changes the state only for peer-groups listed in the constants.yml dictionary (and only for address families from the peer-group value).
**- How to verify it**
Initially, when we start SONiC FRR has BBR enabled for PEER_V4 and PEER_V6:
```
admin@str-s6100-acs-1:~$ vtysh -c 'show run' | egrep 'PEER_V.? allowas'
neighbor PEER_V4 allowas-in 1
neighbor PEER_V6 allowas-in 1
```
Then we apply following configuration to the db:
```
admin@str-s6100-acs-1:~$ cat disable.json
{
"BGP_BBR": {
"all": {
"status": "disabled"
}
}
}
admin@str-s6100-acs-1:~$ sonic-cfggen -j disable.json -w
```
The log output are:
```
Oct 14 18:40:22.450322 str-s6100-acs-1 DEBUG bgp#bgpcfgd: Received message : '('all', 'SET', (('status', 'disabled'),))'
Oct 14 18:40:22.450620 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-f', '/tmp/tmpmWTiuq']'.
Oct 14 18:40:22.681084 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V4 soft in']'.
Oct 14 18:40:22.904626 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V6 soft in']'.
```
Check FRR configuraiton and see that no allowas parameters are there:
```
admin@str-s6100-acs-1:~$ vtysh -c 'show run' | egrep 'PEER_V.? allowas'
admin@str-s6100-acs-1:~$
```
Then we apply enabling configuration back:
```
admin@str-s6100-acs-1:~$ cat enable.json
{
"BGP_BBR": {
"all": {
"status": "enabled"
}
}
}
admin@str-s6100-acs-1:~$ sonic-cfggen -j enable.json -w
```
The log output:
```
Oct 14 18:40:41.074720 str-s6100-acs-1 DEBUG bgp#bgpcfgd: Received message : '('all', 'SET', (('status', 'enabled'),))'
Oct 14 18:40:41.074720 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-f', '/tmp/tmpDD6SKv']'.
Oct 14 18:40:41.587257 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V4 soft in']'.
Oct 14 18:40:42.042967 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V6 soft in']'.
```
Check FRR configuraiton and see that the BBR configuration is back:
```
admin@str-s6100-acs-1:~$ vtysh -c 'show run' | egrep 'PEER_V.? allowas'
neighbor PEER_V4 allowas-in 1
neighbor PEER_V6 allowas-in 1
```
*** The test coverage ***
Below is the test coverage
```
---------- coverage: platform linux2, python 2.7.12-final-0 ----------
Name Stmts Miss Cover
----------------------------------------------------
bgpcfgd/__init__.py 0 0 100%
bgpcfgd/__main__.py 3 3 0%
bgpcfgd/config.py 78 41 47%
bgpcfgd/directory.py 63 34 46%
bgpcfgd/log.py 15 3 80%
bgpcfgd/main.py 51 51 0%
bgpcfgd/manager.py 41 23 44%
bgpcfgd/managers_allow_list.py 385 21 95%
bgpcfgd/managers_bbr.py 76 0 100%
bgpcfgd/managers_bgp.py 193 193 0%
bgpcfgd/managers_db.py 9 9 0%
bgpcfgd/managers_intf.py 33 33 0%
bgpcfgd/managers_setsrc.py 45 45 0%
bgpcfgd/runner.py 39 39 0%
bgpcfgd/template.py 64 11 83%
bgpcfgd/utils.py 32 24 25%
bgpcfgd/vars.py 1 0 100%
----------------------------------------------------
TOTAL 1128 530 53%
```
**- Which release branch to backport (provide reason below if selected)**
- [ ] 201811
- [x] 201911
- [x] 202006
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