Make zfs-initramfs and zfsutils-linux install optional

Here is the change to the ubuntu desktop seed as an example: https://git.launchpad.net/~ubuntu-core-dev/ubuntu-seeds/+git/ubuntu/commit/?h=eoan&id=00192c2d530e6b9a636ac0dcd754c4a80a2319e5. Ubiquity upload is on the way.

Subscribed infinity for tracking the release. Look at the commit why it's not (yet) added to live-common.

I added zfsutils-linux and zfs-initramfs to filesystem.manifest-remove on the iso (bind mount) and did the following tests:

* Normal zfs installation without the patch: zfs-initramfs is removed (but not zfsutils-linux which is a different issue that needs investigation)
* Normal zfs installation with the patch: both packages are kept installed on target.

(in unapproved)

This bug was fixed in the package ubuntu-meta - 1.439

---------------
ubuntu-meta (1.439) eoan; urgency=medium

  * Refreshed dependencies
  * Only install zfs-initramfs and zfsutils-linux when needed (LP: #1847739)
  * Removed zfs-initramfs from desktop-minimal-recommends [amd64 arm64
    ppc64el s390x], desktop-recommends [amd64 arm64 ppc64el s390x]
  * Removed zfsutils-linux from desktop-minimal-recommends, desktop-
    recommends

 -- Didier Roche <email address hidden> Fri, 11 Oct 2019 12:22:14 +0200

This bug was fixed in the package ubuntustudio-meta - 0.198

---------------
ubuntustudio-meta (0.198) eoan; urgency=medium

  * Refreshed dependencies
  * Removed zfs-initramfs from desktop-core-recommends [amd64 arm64
    ppc64el s390x], desktop-recommends [amd64 arm64 ppc64el s390x]
    (LP: #1847739)
  * Removed zfsutils-linux from desktop-core-recommends, desktop-
    recommends (LP: #1847739)
  * Removed zsys from desktop-core-recommends [amd64 arm64 armhf i386
    ppc64el], desktop-recommends [amd64 arm64 armhf i386 ppc64el]
    (LP: #1847739)

 -- Erich Eickmeyer <email address hidden> Fri, 11 Oct 2019 07:54:57 -0700

This bug was fixed in the package ubiquity - 19.10.19

---------------
ubiquity (19.10.19) eoan; urgency=medium

  [ Sebastien Bacher ]
  * Update the 'Skip' string to '_Skip', matching the change done to
    ubiquity.ui some years ago. The template needs to be manual updated which
    wasn't done at the time, should make it translatable again (LP: #1731070)

  [ Mathieu Trudel-Lapierre ]
  * Update translations from Launchpad.
  * debian/real-po/templates.pot: update template using debconf-updatepo.

  [ Jean-Baptiste Lallement ]
  * Switch zpool sync property back to standard after installation
  * zfsutils-linux and zfs-initramfs moved to the live task, don't remove them
    at the end of installation (LP: #1847739)
  * Change partition types to Solaris Boot and Root types

  [ Didier Roche ]
  * As we are not going to own in the end org.zsys, move our identifier tags
    to com.ubuntu.zsys (LP: #1847711)

 -- Jean-Baptiste Lallement <email address hidden> Fri, 11 Oct 2019 12:05:23 +0200

I went ahead and added ZFS to mate-live, since Martin forgot that step when he fixed his meta.

This bug was fixed in the package ubuntu-mate-meta - 1.257

---------------
ubuntu-mate-meta (1.257) eoan; urgency=medium

  * Refreshed dependencies (LP: #1847739)
  * Removed zfs-initramfs from core-recommends [amd64 arm64 ppc64el
    s390x], desktop-recommends [amd64 arm64 ppc64el s390x]
  * Removed zfsutils-linux from core-recommends, desktop-recommends
  * Removed zsys from core-recommends [amd64 arm64 armhf i386 ppc64el],
    desktop-recommends [amd64 arm64 armhf i386 ppc64el]

 -- Martin Wimpress <email address hidden> Sat, 12 Oct 2019 18:46:55 +0100

This bug was fixed in the package xubuntu-meta - 2.231

---------------
xubuntu-meta (2.231) eoan; urgency=medium

  * Refreshed dependencies (LP: #1847739)
  * Removed zfs-initramfs from core-recommends [amd64 arm64 ppc64el
    s390x], desktop-recommends [amd64 arm64 ppc64el s390x]
  * Removed zfsutils-linux from core-recommends, desktop-recommends
  * Removed zsys from core-recommends [amd64 arm64 armhf i386 ppc64el],
    desktop-recommends [amd64 arm64 armhf i386 ppc64el]

 -- Adam Conrad <email address hidden> Sat, 12 Oct 2019 14:15:15 -0600