/etc/grub.d/10_linux_zfs fail when zpool import output to stdout

## 1. Symptoms, `update-grub` fails with "no such pool"

### output
 benjamin@x250:~$ sudo update-grub
 Sourcing file `/etc/default/grub'
 Sourcing file `/etc/default/grub.d/init-select.cfg'
 Generating grub configuration file ...
 cannot open 'This': no such pool

### debug output
 benjamin@x250:~$ sudo update-grub
 Sourcing file `/etc/default/grub'
 Sourcing file `/etc/default/grub.d/init-select.cfg'
 Generating grub configuration file ...
 + set -u
 + which zfs
 +
 + imported_pools=
 + mktemp -d /tmp/zfsmnt.XXXXXX
 + MNTDIR=/tmp/zfsmnt.bunqET
 + mktemp -d /tmp/zfstmp.XXXXXX
 + ZFSTMP=/tmp/zfstmp.X21h0R
 + RC=0
 + trap on_exit EXIT INT QUIT ABRT PIPE TERM
 + GRUB_LINUX_ZFS_TEST=
 + import_pools
 + zpool list
 + awk {if (NR>1) print $1}
 + local initial_pools=x250
 + local all_pools=
 + local imported_pools=
 + zpool import -f -a -o cachefile=none -N
 + imported_pools=This pool uses the following feature(s) not supported by this system:
  com.delphix:spacemap_v2 (Space maps representing large segments are more efficient.)
  org.zfsonlinux:project_quota (space/object accounting based on project ID.)
 All unsupported features are only required for writing to the pool.
 The pool can be imported using '-o readonly=on'.
 This pool uses the following feature(s) not supported by this system:
  com.delphix:spacemap_v2 (Space maps representing large segments are more efficient.)
 All unsupported features are only required for writing to the pool.
 The pool can be imported using '-o readonly=on'.
 + on_exit
 + zpool export This
 cannot open 'This': no such pool

## 2. Cause
 Only the stderr of `zpool import -f -a -o cachefile=none -N` is sent to /dev/null so when the
 command outputs on stdout everything ends up in the `imported_pools` variable

## 3. Quick fix
 in /etc/grub.d/10_linux_zfs:67, replace:
  zpool import -f -a -o cachefile=none -N 2>/dev/null
 by
  zpool import -f -a -o cachefile=none -N >/dev/null 2>&1

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: grub-common 2.04-1ubuntu12
ProcVersionSignature: Ubuntu 5.3.0-19.20-generic 5.3.1
Uname: Linux 5.3.0-19-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu8
Architecture: amd64
CurrentDesktop: XFCE
Date: Sat Oct 26 17:17:20 2019
SourcePackage: grub2
UpgradeStatus: Upgraded to eoan on 2019-10-26 (0 days ago)
modified.conffile..etc.grub.d.10_linux_zfs: [modified]
mtime.conffile..etc.grub.d.10_linux_zfs: 2019-10-26T17:14:38.665289