Make secure.yml idempotent

Hi,

I have a use-case where I use Ansibles.mysql inside Vagrant, and where I may need to run `vagrant provision` several times. The provisioning kept failing on this step, because between the first run and the next ones, the root password had been edited to be the non-empty default value.

I think providing explicit values for the tasks in `secure.yml` while ignoring connection errors for two specific tasks (which are meant to be run only once, really) is a good way to make the role idempotent.