mon: OSDMonitor: split 'osd pool set' out of 'prepare_command'

We should start doing this across the whole 'prepare_command' function.
Makes it prettier to the reader, and easier to add new code.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: allow (un)setting 'hashpspool' flag via 'osd pool set'

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