chassisdb.conf path has to be sourced from platform directory in docker-database

[BrynXu]

Description
On a linecard, chassisdb.conf will not be present in /etc/sonic because
this is a condition to start docker-database-chassis. However we still
want to source this file and proceed to create database_global.json, add
redis_chassis.server to the container and to not call
update_chassisdb_config.
This is a bug on VoQ modular switch only.

Steps to reproduce the issue:
The problem will happen on VoQ modular switch linecard once Sonic boots up.

Describe the results you received:
Missing chassis_db info in database_config.json in VoQ linecard.

Describe the results you expected:
database_config.json in VoQ linecard should be the same as the database_config.json in supervisor.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

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