Wait for mysqld to be up before attempting restore.

[enisoc]

This avoids crash-looping when starting vttablet and mysqld simultaneously.

Fixes #2397.

cc @derekperkins

[derekperkins]

This looks great. Does this solve the problem completely or should we reconfigure the InitContainer to use the init-config option from #3530?

[enisoc]

I think we'll still need to somehow wait for my.cnf to be generated. I'm concerned that using init_config in an init container implies that we have to keep the env vars and mounted config files in sync between that init container and the mysqlctld container, or else they will generate different things.

If we instead make mysqlctld the only thing that ever generates my.cnf, we can have a single source of truth. We just need to make vttablet wait gracefully for my.cnf to show up, like this PR does for mysql.sock.

[alainjobart]

LGTM