[bgpcfgd]: Fixes for BBR

[pavel-shirshov]

- Why I did it
The current implementation of BBR has two issues:

1. constants.yml uses key "bgp.bbr.enabled" as a key to enable disable BBR feature and as a default state for the BBR feature.
2. If the current configuration doesn't have some peer-groups, which are listed in constants.yml bgp.peers.*.bbr bgpcfgd will report an error of updating such peer-groups.

- How I did it

1. I added a key bgp.bbr.default_state with values "enabled" or "disabled" into constants.yml. Now bgp.bbr.enabled controls will bgpcfgd support BBR feature or not. And bgp.bbr.default_state control initial state of the BBR.
2. I added an extra-code to check what peer-groups in the current config. After that bgpcfgd skips all peer-groups from constants,yml which are not presented in bgp config.

- How to verify it
Build an image and check the behaviour

admin@str-s6100-acs-1:~$ sonic-cfggen -j disable.txt -w    
admin@str-s6100-acs-1:~$ vtysh -c 'show run' | grep allow      
admin@str-s6100-acs-1:~$ sonic-cfggen -j enable.txt -w     
admin@str-s6100-acs-1:~$ vtysh -c 'show run' | grep allow
  neighbor PEER_V4 allowas-in 1
  neighbor PEER_V6 allowas-in 1

Nov 18 22:26:29.815909 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'show running-config']'.
Nov 18 22:26:30.038863 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-f', '/tmp/tmp5o11zi0e']'.
Nov 18 22:26:30.258212 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V4 soft in']'.
Nov 18 22:26:30.476447 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V6 soft in']'.
Nov 18 22:27:00.247345 str-s6100-acs-1 DEBUG bgp#bgpcfgd: Received message : '('all', 'SET', (('status', 'enabled'),))'
Nov 18 22:27:00.247853 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'show running-config']'.
Nov 18 22:27:00.476119 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-f', '/tmp/tmp_pptzt_j']'.
Nov 18 22:27:00.700638 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V4 soft in']'.
Nov 18 22:27:00.920070 str-s6100-acs-1 DEBUG bgp#bgpcfgd: execute command '['vtysh', '-c', 'clear bgp peer-group PEER_V6 soft in']'.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)