secchan: Drop configuration file support.

secchan: Drop configuration file support.

Configuration file support was added to secchan specifically to allow
vswitchd to change the set of NetFlow collectors at runtime without killing
and restarting secchan.  secchan is integrated into vswitchd, so vswitchd
can now do this through a function call instead of through a configuration
file.  This means that we can kill off the secchan configuration file
and add back the --netflow option that it replaced.

Also, add a --mgmt-id option to supplant the other use for the
configuration file that had appeared in the meantime.