Let PMM client wait for mysql.sock
Let PMM client wait for mysql.sock

There is a race condition where PMM can come up before the `mysql.sock` file is ready.  PMM will fail to start and then permanently crash.  Instead, wait for the sock file to be available and then proceed with PMM initialization.

Also, latest fix to the `vitess/pmm-client` image means we no longer need the `|| true` fix for `pmm-admin add`.

Signed-off-by: Mark Solters <48446e04050b842d4cfb37e68372dd70ae8e791d@gmail.com>

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