OpenZFS 9185 - Enable testing over NFS in ZFS performance tests

This change makes additions to the ZFS test suite that allows the
performance tests to run over NFS. The test is run and performance data
collected from the server side, while IO is generated on the NFS client.

This has been tested with Linux and illumos NFS clients.

Authored by: Ahmed Ghanem <ahmedg@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Kevin Greene <kevin.greene@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>

OpenZFS-issue: https://www.illumos.org/issues/9185
Closes #8367

tests/zfs-tests/include/commands.cfg

@@ -100,6 +100,7 @@ export SYSTEM_FILES='arp
     readlink
     rm
     rmdir
+    scp
     sed
     seq
     setenforce
@@ -110,6 +111,7 @@ export SYSTEM_FILES='arp
     shuf
     sleep
     sort
+    ssh
     stat
     strings
     su