sflow: Always add poller and sampler together.

sflow: Always add poller and sampler together.

he ofproto_sflow_add_poller() and ofproto_sflow_add_sampler() calls
should always be made together, either when a port is added
dynamically with ofproto_sflow_add_port() or when the sflow_agent is
first created in ofproto_sflow_set_options().  I was seeing odd
behavior where either the pollers or the samplers would never be
instantiated depending on the order that things happened.  (It's OK to
add the same sampler or poller again, because the library routines
sfl_agent_addPoller() and sfl_agent_addSampler() will just return the
existing one if it is there.  Perhaps we should add a comment to make
that clear?).

I changed the parameters to ofproto_sflow_add_sampler to make it work
the same way as ofproto_sflow_add_poller, where the options are
extracted from os->options within the function itself.