disk: new command, with list, prepare, activate

Create new disk command.

Change 'osd prepare ...' to 'disk prepare ...'
Change 'osd activate ...' to 'disk activate ...'
Add 'disk list HOST'

This keeps the osd command simple and clean, and makes 'disk list' make
sense.

The HOST vs HOST:DISK is slightly weird still.  Oh well.

Signed-off-by: Sage Weil <sage@inktank.com>