prog/main: use flags.app.port for default target

[schu]

User can control the webserver listen address with -app.http.address.
If they do, scope --mode probe ... should use the port part of address
in the default target and not ':4040'. Otherwise we encounter a
dial tcp 127.0.0.1:4040: getsockopt: connection refused error and
scope-probe cannot report.

Signed-off-by: Michael Schubert michael@kinvolk.io

[schu]

Not perfect as the scope launcher script always prints 4040 as the listening port (https://github.com/weaveworks/scope/blob/master/scope#L178-L180), but allows you to actually use a different -app.http.address locally.

[2opremio]

Thanks @schu

Not perfect as the scope launcher script always prints 4040 as the listening port (https://github.com/weaveworks/scope/blob/master/scope#L178-L180), but allows you to actually use a different -app.http.address locally.

It wasn't perfect before either (i.e. you could already listen on an different address)

[2opremio]

Related: #1190