datapaths: Improve log message

Log message now indicates that it's attempting to connect and the address
it's attempting to connect to.
datapaths: Rename address arg; add default address

The address argument on the commandline used to be "addr", which is not
consistent with elsewhere in POX.  Changed to "address".

Also, address now defaults to 127.0.0.1.  I'm not positive this is a good
idea.  Unless you're careful (--no-openflow), this means it'll probably
connect to itself.