mon: implement 'mds set max_mds|max_file_size'

First, add the ability to modify max_file_size.  While we are at it, move
to a more sensible interface for adjusting max_mds too.

Signed-off-by: Sage Weil <sage@inktank.com>
mds/MDSMap: include max_file_size in plaintext print output

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1071 from ceph/wip-max-file-size

allow mds max file size to be adjusted

Reviewed-by: Yan, Zheng <zheng.z.yan@intel.com>