mon/PGMonitor: fix 'pg dump_[pools_]json'

Use the correct type for the dumpcontents arg.  Fixes the dump_pools_json
output and avoids these errors:

2013-07-29 13:09:14.089188 7fa0c5d21700 -1 0x7fa0c5d1e7a8
2013-07-29 13:09:16.306560 7fa0c5d21700 -1 bad boost::get: key dumpcontents is not type std::vector<std::string, std::allocator<std::string> >
2013-07-29 13:09:16.317104 7fa0c5d21700 -1 0x7fa0c5d1e7a8
2013-07-29 13:09:16.317136 7fa0c5d21700 -1 bad boost::get: key dumpcontents is not type std::vector<std::string, std::allocator<std::string> >

Fixes: #5786
Signed-off-by: Sage Weil <sage@inktank.com>