Update init script to allow /dev/disk/by-id import

Many people prefer to use by-id at import time instead of using
the cache file. This can be a much better solution than the cache
file in some environments so we're adding some infrastructure to
allow it.

This functionality is disabled by default.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1929