Add script for testing BGP allow list (#2572)

What is the motivation for this PR?
The BGP allow list feature was introduced in SONiC. This PR is to add a script for testing the BGP allow list feature.

How did you do it?
Add a new script for testing BGP allow list. Covered scenarios:

* Ensure that constants.bgp.allow_list.default_action is "permit". No BGP allow list is configured.
  * Announce routes with and without test community '1010:1010' to the first T0 VM by exabgp.
  * Check routes on the first T0 VM. All the routes should be successfully injected.
  * Check routes on DUT. All the routes should be accepted by DUT.
  * Check routes on other T0 and T2 VMs. All the routes should be advertised by DUT. The drop_community defined in 
  * /etc/sonic/constants.yml should be added to all routes. The original community of routes should be kept.

* Ensure that constants.bgp.allow_list.default_action is "permit". BGP allow list is configured.
  * Announce routes with and without test community '1010:1010' to the first T0 VM by exabgp.
  * Check routes on the first T0 VM. All the routes should be successfully injected.
  * Check routes on DUT. All the routes should be accepted by DUT.
  * Check routes on other T0 and T2 VMs. All the routes should be advertised by DUT. The drop_community should only be   
  * added to routes not on allow list. The original community of routes should be kept.

* Ensure that constants.bgp.allow_list.default_action is "deny". No BGP allow list is configured.
  * Announce routes with and without test community '1010:1010' to the first T0 VM by exabgp.
  * Check routes on the first T0 VM. All the routes should be successfully injected.
  * Check routes on DUT. All the routes should be accepted by DUT.
  * Check routes on other T0 and T2 VMs. No routes should be advertised by DUT.

* Ensure that constants.bgp.allow_list.default_action is "deny". BGP allow list is configured.
  * Announce routes with and without test community '1010:1010' to the first T0 VM by exabgp.
  * Check routes on the first T0 VM. All the routes should be successfully injected.
  * Check routes on DUT. All the routes should be accepted by DUT.
  * Check routes on other T0 and T2 VMs. Only the routes on allow list should be advertised by DUT. No drop_community should be added to advertised routes. The original community of routes should be kept.

Relevant change: sonic-net/sonic-buildimage#5309

How did you verify/test it?
Run the test script using latest master image. Currently some test cases can't pass because of issue: sonic-net/sonic-buildimage#6001

If add `on-match next` to /usr/share/sonic/templates/bgpd/templates/general/policies.conf.j2 and restart bgp service, then all the cases can pass:
```
route-map FROM_BGP_PEER_V6 permit 1
 on-match next
 set ipv6 next-hop prefer-global
```
Any platform specific information?
No

Supported testbed topology if it's a new test case?
This test only supports topology type t1.

Signed-off-by: Xin Wang <xiwang5@microsoft.com>