mon: OSDMonitor: move 'osd pool get' handling to preprocess_command()

It's a read operation; there's no need for it to be in prepare_command().

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: add formatted output to 'osd pool get'

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>