virtual-chassis test uses multiple vs instances to simulate a
modular switch and a redis-chassis service is required to run on
the vs instance that represents a supervisor card.
This change allows vs docker start redis-chassis service according
to external config file.
**- Why I did it**
To support virtual-chassis setup, so that we can test distributed forwarding feature in virtual sonic environment, see `Distributed forwarding in a VOQ architecture HLD` pull request at https://github.com/Azure/SONiC/pull/622
**- How I did it**
The sonic-vs start.sh is enhanced to start new redis_chassis service if external chassis config file found. The config file doesn't exist in current vs environment, start.sh will behave like before.
**- How to verify it**
The swss/test still pass. The chassis_db service is verified in virtual-chassis topology and tests which are in following PRs.
Signed-off-by: Honggang Xu <hxu@arista.com>
(cherry picked from commit c1d45cf81ce3238be2dcbccae98c0780944981ce)
Co-authored-by: Honggang Xu <hxu@arista.com>
