Add proxysql-admin tool

[tmirks]

Would it make sense to integrate with Percona's proxysql-admin tool so that proxysql-cluster works with Galera / PXC?

[jmdejager]

Great idea, that would be awesome! How do you think we could best integrate this @tmirks?
Maybe something like an init_percona command and an init_mysql command and run the container with
--dbprovider percona ?

[tmirks]

@jmdejager We're actually maintaining our own "downstream fork" loosely based on the work done here.

Our version already has proxysql-admin built in and we're using a postStart lifecycle hook that:

1. Updates /etc/proxysql-admin.cnf
2. Runs proxysql-admin --enable

It's pretty specific to how we want it to work (e.g. we're using the --use-existing-monitor-password flag) so when I have some time I'll try to integrate with this project on a branch.

We also have our own Dockerfile that installs ProxySQL from Percona's packages; would it be OK to use Percona's packages instead of the official ProxySQL packages?