LU-1199 build: Reintroduce RPMBUILD_WITH_ZFS

Prakash pointed out that RPMBUILD_WITH_ZFS got lost in the landing of:

  LU-3117 build: zfs-0.6.1 kmod+dkms compatibility

In a rebase, the above commit should be moved earlier than the LU-1199
patches, and then this can be merged into the lustre spec file overhaul
commit.

Change-Id: I20f54ee5788874ec31e4ee0b8cdbc687c13ad76b
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>