rbd: mark optional positional arguments as such in help output

rbd: mark optional positional arguments as such in help output

Currently at least five commands have optional positional arguments.

Overloading po::value<std::string>()->default_value("") for this
is a bit sneaky but nothing better fits into the existing Shell.cc
framework.

Note that strictly speaking "[<interval>] [<start-time>]" should be
"[<interval> [<start-time>]]" but we aren't doing that here because
"ceph" command doesn't do it either.

Fixes: https://tracker.ceph.com/issues/54191
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>