nsqlookupd: pass defaults from opts into flags

[mreiferson]

This updates nsqlookupd so that default values from the options struct
are passed into flag defaults since all flag values implement flag.Getter
and the recent change to go-options (mreiferson/go-options#15) was insufficient.

RFR @jehiah
cc @ploxiln

Fixes #826

[ploxiln]

I can confirm that this fixes the default broadcast address for nsqlookupd.

Check out the interface specifier on that ipv6 link-local address (it's pretty cool that this ipv6 stuff works):

[nsqlookupd] 2016/12/29 01:53:56.504125 200 GET /channels?topic=t1 ([fe80::3e15:c2ff:fec0:28a8%en0]:50923) 9.42µs

I'll separately look into why there was no error printed when no nsqlookupd could be contacted.

[mreiferson]

I'll separately look into why there was no error printed when no nsqlookupd could be contacted.

When nsqlookupd returned an empty broadcast address nsqd would skip it entirely, see https://github.com/nsqio/nsq/blob/master/nsqd/lookup.go#L188

[ploxiln]

Yeah I figured something like that.

Options I was thinking of:

• log error if there was at least one lookupd address configured, but none could be contacted
• log error if broadcast address is empty (right where you point out)
• just use tcp address (without port) if blank broadcast address

[ploxiln]

fixes my test case

[mreiferson]

@jehiah 🍏 💚 📗