Replace custom zpool configs with generic configs

Replace custom zpool configs with generic configs

To streamline testing I have in the past added several custom configs
to the zpool-config directory.  This change reverts those custom configs
and replaces them with three generic config which can do the same thing.
The generic config behavior can be set by setting various environment
variables when calling either the zpool-create.sh or zpios.sh scripts.

For example if you wanted to create and test a single 4-disk Raid-Z2
configuration using disks [A-D]1 with dedicated ZIL and L2ARC devices
you could run the following.

$ ZIL="log A2" L2ARC="cache B2" RANKS=1 CHANNELS=4 LEVEL=2 \
  zpool-create.sh -c zpool-raidz

$ zpool status tank
  pool: tank
 state: ONLINE
 scan: none requested
config:

      NAME        STATE     READ WRITE CKSUM
      tank        ONLINE       0     0     0
        raidz2-0  ONLINE       0     0     0
          A1      ONLINE       0     0     0
          B1      ONLINE       0     0     0
          C1      ONLINE       0     0     0
          D1      ONLINE       0     0     0
      logs
        A2        ONLINE       0     0     0
      cache
        B2        ONLINE       0     0     0

errors: No known data errors