Introduce JAILED_DATASET option to poudriere.conf

[arrowd]

Rationale for the change:

We at $WORK are using Poudriere as a part of the CI process. For a selected set of ports we define WITH_TESTING_PORTS, which makes the test target a part of the pipeline. It works quite well for many of ports we're using.

However, one of the projects we've developed requires creating a ZFS dataset as part of its tests. The #1175 PR made it possible for zfs command to run, but no datasets or pools are visible from within the jail. This follow-up change passes through a throw-away ZFS dataset into the jail and gives full access to it.

I understand that the feature is quite niche, but it doesn't add much complexity to the code, so I hope it will be accepted.

[bdrewery]

Could we add some hooks, or use existing hooks, to achieve this?

[arrowd]

I agree, the hooks feels like a more proper place for this feature, but from what I gather, even the "jail start" hook is ran after the jail is actually created, so there is no way for it to pass allow.mount=1 allow.mount.zfs=1 enforce_statfs=1.