common: config: let us obtain a diff between current and default config

It's mildly annoying when trying to figure out what has been changed on
a running system's config options and having to rely on whatever is set
on ceph.conf and the admin's memory of what has been injected.

With this we can simply ask the daemon for the diff between what would be
its default and what is its current config.

Current form will output extraneous information that was not directly
supplied by the user though, such as 'host' 'fsid' and 'daemonize', as
well as defaults we may rewrite ourselves (leveldb tunables on the monitor
for instance).  Nonetheless, it's way better than the alternative and
considering it should be used solely for debug purposes I think we can
get away with it.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
common: ceph_context: add admin socket command 'config diff'

Outputs a diff between the current config and what the daemon believes
to be its default config.

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