[swss]: Clear MUX-related state DB tables on start

[theasianpianist]

Signed-off-by: Lawrence Lee lawlee@microsoft.com

- Why I did it
Port configuration changes can leave keys for unused interfaces behind in STATE DB

- How I did it

• Add MUX_CABLE_TABLE to set of tables to clear on SWSS start, which
  will clear HW_MUX_CABLE_TABLE and MUX_CABLE_TABLE
• Order swss to start before pmon to ensure that DBs are cleared before
  xcvrd (running inside pmon) starts and re-populates the tables

- How to verify it

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[prsunny]

cleanup table lgtm.. @vdahiya12 , @lguohan to check service dependency.

[lguohan]

@jleveque , now pmon is affected by swss.

[jleveque]

@jleveque , now pmon is affected by swss.

Thanks for the heads-up! Do we have any concerns that SwSS startup issues could prevent PMon from starting, thus causing thermal overload issues? I believe since this is only a Before= dependency, PMon should still start up even if SwSS fails. Can we confirm this?

[theasianpianist]

@jleveque This dependency should not prevent PMON from starting if SWSS fails, since Before= does not enforce any dependencies on the health of the SWSS service, it only waits until SWSS has either completely started or failed: https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Before=

[theasianpianist]

/Azurepipelines run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[lguohan]

in the chassis supervisor card environment, the pmon needs to before swss.