Add zpool events tests

Add zpool events tests

* events_001_pos - Verify the expected events are generated when
  invoking the various zpool sub-commands.  These events must
  appear in `zpool event` and be consumed by the ZED.

* events_002_pos - Verify the ZED consumes events which were
  generated while it wasn't running when it is started.
  Additionally, verify that events are only processed once.

As part of this change the default.cfg used by the test suite
was changed to a default.cfg.in file.  This was needed so the
install location of all zed scripts, not only the enabled ones,
could be reliably determined.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6128