lxd/storage/drivers/driver/zfs: Adds ensureInitialDatasets function
lxd/storage/drivers/driver/zfs: Adds ensureInitialDatasets function

And calls it during Create() and Mount() to ensure all initial datasets always exist
and are configured using current policy.

Also switches to Reverter in Create().

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,lxc/lxd