Add zconfig test for adding and removing vdevs

Add zconfig test for adding and removing vdevs

This test performs a sanity check of the zpool add and remove commands.  It
tests adding and removing both a cache disk and a log disk to and from a zpool.
Usage of both a shorthand device path and a full path is covered.  The test
uses a scsi_debug device as the disk to be added and removed.  This is done so
that zpool will see it as a whole disk and partition it, which it does not
currently done for loopback devices.  We want to verify that the manipulation
done to whole disks paths to hide the parition information does not break the
add/remove interface.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>