mon/OSDMonitor: fix 'osd pool set ...' int/float parsing

In 2fe0d0d97af95c22db80800f5b9da51f672d9407 I assumed that the command may
come with a range of JSON formats (string, int, float), but in reality the
client gets the schema via get_command_descriptions regardless of what
version it is running.  That means we only need to handle the string case. 
This fixes warnings like

2013-10-29 09:04:23.911996 7f38df7fe700 -1 bad boost::get: key val is not
type long 2013-10-29 09:04:23.934971 7f38df7fe700 -1 0x7f38df7f9578 
2013-10-29 09:04:23.935014 7f38df7fe700 -1 bad boost::get: key val is not
type float 2013-10-29 09:04:23.946177 7f38df7fe700 -1 0x7f38df7f9578

which are noted in #6673.

Signed-off-by: Sage Weil <sage@inktank.com>