pybind: enable --keyword=arguments in ceph_argparse

pybind: enable --keyword=arguments in ceph_argparse

This is a simple implementation that treats anything
that matches the "--X=Y" pattern as separate from
positional arguments.

This works well for optional arguments.  Mandatory
arguments still need to be specified positionally,
or the parsing code will think the command's
argument description has not been satisfied.

Signed-off-by: John Spray <john.spray@redhat.com>