Do not try to resolve DNS for OpenFlow controllers or netflow collectors.

Do not try to resolve DNS for OpenFlow controllers or netflow collectors.

Until now, setting a netflow collector to a DNS name would cause
secchan to attempt to resolve that DNS name each time that the set of
netflow collectors is re-set.  For the vswitch, this is every time that
the vswitch reconfigures itself.

Unfortunately, DNS lookup within secchan cannot work as currently
implemented, because it needs both an asynchronous DNS resolver library
and in-band control updates.  Currently we have neither.  Attempting to
look up DNS anyway just hangs.

This commit disables DNS lookup entirely, and updates the documentation to
change user expectations.  DNS still won't work, but at least it won't
hang.

Bug #1609.