LU-12677 tests: conf-sanity test_21d must keep zpool

LU-12677 tests: conf-sanity test_21d must keep zpool

Currently test_21d destroys the zpool during writeconf_or_reformat
but the zpool must be preserved for writeconf to work as intended.

Cray-bug-id: LUS-7688
Test-Parameters: fstype=zfs testlist=conf-sanity
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I8fdc123504b70e59a9ca789141e56815377f6b35
Reviewed-on: https://review.whamcloud.com/35848
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>