mgr: add generic --help output

mgr: add generic --help output

Creating an MgrStandby in order to call its usage() method has
two problems:

1) MgrStandby wants to connect to a running cluster; if the
   cluster isn't up, you'll be left staring at a blinking cursor.
2) MgrStandby::usage() doesn't print anything anyway.

This commit takes pretty much the same approach to printing
usage information as ceph_mon.cc, ceph_osd.cc and ceph_mds.cc.

Signed-off-by: Tim Serong <tserong@suse.com>