Feature don't delete server

[jgnagy]

Allow choosing to persist the rancher server beyond restarts of the container. Without this option (and setting it to false), restarting the host will cause the server (and its MySQL DB) to be recreated. This tends to upset people.

We might consider updating the README if this PR is accepted.

[jgnagy]

Looks like your beaker / testing requirements need a newer Ruby.