[veos_vtb] Add connection variable to 'sonic' group (#2639)

By default, Ansible will search for group variables from the directory
where inventory file locates. For vtestbed Pytest users who calls Pytest
with `tests/veos_vtb`, Ansible fails to find the groups variables
defined under `ansible/group_vars`, thus fail to use `multi_passwd_ss`.
So let's add those connection variables directly to `veos_vtb`

Signed-off-by: Longxiang Lyu <lolv@microsoft.com>

ansible/veos_vtb

@@ -54,6 +54,8 @@ all:
  sonic:
  vars:
  mgmt_subnet_mask_length: 24
+ ansible_connection: multi_passwd_ssh
+ ansible_altpassword: YourPaSsWoRd
  hosts:
  vlab-01:
  ansible_host: 10.250.0.101