Add "config get <var>" to the admin socket

This adds the possibility to get the value of one specific configuration setting instead of dumping them all.

This prevents you from parsing the JSON first in search of your config value.

For admins it's also easier, they don't have to pipe through grep first.

$ ceph --admin-socket /path/to/socket config get debug_osd

The second patch fixes some whitespace issues in the same file, although the whole file seems to need a whitespace / indentation fixup.