sso_auth: make provider_*_okta_server optional

[kjetijor]

Problem

Quickstart (and testing in practice) seems to indicate that PROVIDER_*_OKTA_SERVER should be optional, if you wish to use the default okta authorization server.

If you are not using the default Okta authorization server you will also need to add PROVIDER_OKTAQUICKSTART_SERVER_ID= to the above file.

Solution

Make it optional!

Notes

[codecov]

Codecov Report

Merging #292 into master will increase coverage by 0.06%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   61.87%   61.94%   +0.06%     
==========================================
  Files          57       57              
  Lines        4640     4638       -2     
==========================================
+ Hits         2871     2873       +2     
+ Misses       1556     1553       -3     
+ Partials      213      212       -1     

Impacted Files	Coverage Δ
internal/auth/configuration.go	47.41% <ø> (+0.44%)	⬆️
internal/auth/providers/okta.go	60.65% <80.00%> (+0.81%)	⬆️

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[Jusshersmith]

Hi @kjetijor,

Thank you contributing and fixing this!