[202012] Migrate missed config on cross branch warm upgrade to 202012

[vaibhavhd]

What I did

Warm upgrade from 201811 (or 201911) to 202012 branch generates config which does not contain some new tables, config attributes introduced in 202012 branch.

1. Missing tables are RESTAPI, TELEMETRY and CONSOLE_SWITCH.
2. Missing attr is synchronous_mode in METADATA
3. Missing VXLAN entries in APPDB. This one cause warmboot failure. Fix warmboot issue from 201811 to 202012. Cause of issue:

201811 image does not have vxlanmgrd. Thus, during warmboot from 201811 to 202012, vxlanmgrd does not do warmstart and reconciliation as restore_count for vxlanmgrd was not set in 201811 image. Since nothing is reconciled to appdb, vxlanorch has nothing to process. Hence, orchagent bake process misses VXLAN_TUNNEL* entries.
Consequently, syncd reports differences between CURRENT and TEMP view and attempts to remove VXLAN entries in new image. This remove operation fail and swss, syncd both crash.

How I did it

Advanced db_migrator version on 202012 branch to handle the new migration logic.

How to verify it

Tested on physical device and executed unit tests locally.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[stephenxs]

Hi @yxieca @vaibhavhd
db_migrator runs not only during upgrading but also during the configuration initialization.
On a switch installed from ONIE, the TELEMETRY table will also be inserted to the CONFIG_DB but no keys generated by default. Just want to clarify, is it the user's responsibility to generate the keys in this case?
Thanks.