exec/java task drivers use host's /etc/hosts even in bridge mode

[tgross]

Coming out of #8900, #8600, and #10766, I noticed that the exec (and java) drivers are simply copying over the host's /etc/hosts, /etc/hostname, and /etc/resolv.conf files, even if we have network.mode = "bridge" set and the task is in a Nomad-owned network namespace.

We probably can't fix this in a totally backwards-compatible way without adding a bunch more flags, but arguably anyone relying on the existing behavior is getting incorrect behavior anyways.