mon: move supported_commands fields, methods into Monitor, and fix leak

We were leaking the static leader_supported_mon_commands.  Move this into
the class so that we can clean up in the destructor.

Rename get_command_descriptions -> format_command_descriptions.

Fixes: #7009
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #954 from ceph/wip-7009

mon: move supported_commands fields, methods into Monitor, and fix leak

Reviewed-by: Greg Farnum <greg@inktank.com>