ZTS: Don't include zpool_create.shlib in zpool_add

ZTS: Don't include zpool_create.shlib in zpool_add

The zpool_add tests include zpool_create.shlib for a few silly
variables.

Don't use those variables for the file names. Include zpool_add.kshlib
for whatever variables we still need.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #9997