zfs test suite: move definition of DISK to the cfg file in zpool_get

The variable is used not only by the setup script but also by the
atf test bodies.

This should have been in r331001.

tests/sys/cddl/zfs/tests/cli_root/zpool_get/setup.ksh

@@ -32,6 +32,4 @@
 . ${STF_SUITE}/include/libtest.kshlib
 verify_runnable "both"
 
-DISK=${DISKS%% *}
-
 default_setup $DISK

tests/sys/cddl/zfs/tests/cli_root/zpool_get/zpool_get.cfg

@@ -69,3 +69,5 @@ typeset -a properties=(
     "feature@skein"
     # "feature@edonr" Edonr is not yet implemented on FreeBSD
 )
+
+export DISK=${DISKS%% *}