Fix 404 redirect

[sblaisot]

This PR fixes 404 redirect by replacing hardcoded redirect URI with configured redirect URI, should it exist in configuration.

I also simplified how redirect_uri is retrieved from configuration with getter and default value.

[shankari]

Code changes look fine to me.

@sblaisot thanks for the contribution!
Can you also indicate the testing done, potentially with a small video since there is not a unit test for this functionality?

[shankari]

For my own edification, I went back to see why I had hardcoded the URL after adding in the functionality to read it from the config. The related commit is 964ed28 in #854
and I don't see any reason why.

So once the testing is done, we can go ahead and commit this.

[sblaisot]

Hi @shankari

I just added unit test for the 404 redirect

[sblaisot]

Launching this test on current master:

F
======================================================================
FAIL: test404Redirect (netTests.TestWebserver.TestWebserver)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/openpath/e-mission-server/emission/tests/netTests/TestWebserver.py", line 57, in test404Redirect
    self.assertEqual(response.get_header("Location"), "http://somewhere.else")
AssertionError: 'https://www.nrel.gov/transportation/openpath.html' != 'http://somewhere.else'
- https://www.nrel.gov/transportation/openpath.html
+ http://somewhere.else


----------------------------------------------------------------------
Ran 1 test in 0.786s

Running this test on this PR's source branch:

.
----------------------------------------------------------------------
Ran 1 test in 0.751s

OK

[shankari]

@aGuttman not sure if you have written the unit test for #886 yet, but @sblaisot has already written the first unit test for the API, so you should be able to add yours to this file as well.