Allow configuration of network namespace container "extra_hosts"

When using the pause container as the parent namespace container for tasks to join their network on, it becomes important to have the ability to control the `/etc/hosts` file so that services can continue to use a hostname as the connection string for their upstreams. This can be done by configuring "extra_hosts" for the docker driver, but we don't seem to have the ability to control the configuration of the pause container.

Since other tasks join the pause container's network, they are not allowed to use "extra_hosts" as they get the error `"Conflicting options: custom host-to-IP mapping and the network mode."`.