Assorted fixes for the performance tests

• Bail out early if we're running the perf tests and forget to specify disks.
• Allow perf tests to run with any number of disks.
• Remove weekly vs. nightly settings
• Move variables with common values to perf.shlib
• Use zinject to clear the ARC over export/import
• Fix dbuf cache size calculation

When the meaning of dbuf_cache_max_bytes changed, the performance
test that covers the dbuf cache started to fail. The test would try to
write files for the test using the max possible size of the cache,
inevitably filling the pool and failing. This change uses
dbuf_cache_shift to correctly calculate the dbuf cache size.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #12408