Troubles replicating the mysql.users table

[fiorix]

The current entry point script always delete the mysql.user table and provisions the root user. This works well for bootstrapping the cluster but once the first node comes online, it tries to synch the mysql.user table but then the entry point script deletes it.

I've made a patch that does not touch the mysql.user table unless the MYSQL_ROOT_PASSWORD environment variable is set. However, to keep the same semantics of the original MySQL docker image, you have to explicitly set MYSQL_ALLOW_EMPTY_PASSWORD like you'd normally do.

This patch allows for the bootstrap node to set MYSQL_USER, MYSQL_PASSWORD and MYSQL_DATABASE and other nodes to set MYSQL_ALLOW_EMPTY_PASSWORD so they can synch the user, password and database from the bootstrap one.