Contour namespace

[stevesloka]

Fixes #2255 by using the downward api to give the default namespace for configuration variables. They are still able to be overridden/configured as before but are no longer hardcoded to projectcontour.

[codecov]

Codecov Report

Merging #2389 into master will decrease coverage by 0.02%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #2389      +/-   ##
==========================================
- Coverage   78.12%   78.09%   -0.03%     
==========================================
  Files          62       62              
  Lines        5288     5292       +4     
==========================================
+ Hits         4131     4133       +2     
- Misses       1069     1070       +1     
- Partials       88       89       +1     

Impacted Files	Coverage Δ
cmd/contour/serve.go	0.00% <ø> (ø)
cmd/contour/servecontext.go	92.92% <60.00%> (-1.58%)	⬇️

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[jpeach]

This looks good to me. I was a little concerned that CONTOUR_NAMESPACE is used differently in all three cases here, but I suppose that's unavoidable, and I didn't come up with a better alternative.

We should file a separate issue to document this, since there is no documentation for environment variable usages (not even in the --help output).

Finally, @stevesloka let me know if you want to squash and update the commit massage, otherwise I can do that when merging the PR.

[stevesloka]

@jpeach update message? Because of the squash? Bah, this is why I have separate commits to make it clear the changes. I guess I can just rebase them together.