Related issue: https://github.com/github/gh-ost/issues/521
- Add --ssl-cert and --ssl-key options to specify SSL public/private
key files
- Allow combining --ssl-allow-insecure with other --ssl* flags.
`mysql.RegisterTLSConfig` allows combining the corresponding
parameters in the `tls.Config` it receives, so gh-ost should
allow this. I found being able to pass --ssl-allow-insecure along
with --ssl-ca, --ssl-cert, and --ssl-key useful in testing.
- Use the same TLS config everywhere. Since the CLI only supports
a single set of --ssl* configuration parameters, this should be
fine -- `mysql.RegisterTLSConfig` documentation indicates the
TLS config given will not be modified, so it can safely be used
in many goroutines provided we also do not modify it. The previous
implementation did not work when the TLS config was duplicated,
which happens when gh-ost walks up the replication chain trying
to find the master. This is because, when the config is duplicated,
we must call `RegisterTLSConfig` again with the new config. This
config is exactly the same, so it's easiest to side-step the issue
by registering the TLS config once and using it everywhere.
