[processing] Do not override libpq defaults

[strk]

Fixes http://hub.qgis.org/issues/15706
Superceeds #3604

\cc @volaya

[strk]

not sure how to test this, as the function building the connection string is the same building the whole commandline, maybe they should be split to allow for easier unit testing...

[volaya]

There should be a default. Otjherwise, users will have to manually type host and port everytime, so this change will mean a regression in terms of UX.

The part of the code you added to the eecution method is fine, we can keep that, but let's at least have 5432 and 'localhost' as default values, to make it easier to call the algorithm in the most usual case

[strk]

Isn't the connection string derived from existing connections ?
For people like me relying on env variables or defaults, the wrong default gets in the way, rather than simplifying.

[strk]

I've just tried "Import Vector into PostGIS database (new connection)" and I confirm it works fine even if I don't feel any of the Host,Database,User,Password fields (as it then correctly rely on libpq defaults)

[strk]

Actually, I'd drop the "active_schema" default too..

[strk]

I would note that the current defaults would mostly be the libpq defaults for most people, so omitting them would not change the experience of anyone with a default PostgreSQL install. The only difference would be not seeing the defaults written in the form.

With latest commit I made it clear that those parameters are optional

[strk]

Ok I've refactored the code to not change the UX. I've been thinking that a power user would know what to do better than a non-power user. So we're back to the need for a testcase

[strk]

Ok @volaya I've now added unit test for the connection string, moving it to its own function.
Env variables are still not being used to fill default parameters