Create an option to deploy a proxysql on top of an existing PXC

[leo-bacchi]

Create an option to deploy a ProxySQL on top of an existing PXC.
For example: if you already have a 3 node cluster that you were doing some testing and wants to incorporate a ProxySQL into your tests, right now you'd have to destroy your entire cluster and re-deploy with type=proxysql-pxc. This option would allow you to specify a PXC node and deploy ProxySQL on top of it.

There is already an option listed in --help for it, but it hasn't been implemented yet:
--proxysql-pxc-node= Container name of one PXC node--proxysql-pxc-node=